Fixing this spell depends a lot on how much you think people should be able to magically extend life. My temptation is to simply say this:

1) Balm of Chesme can be cast on the same subject for its full duration and full effect a number of times equal to the subject's own Spirit SI (or half Aura attribute if you don't use Spirit skill). Castings beyond that amount simply don't work; and if the spell's duration is cut short, the spell has no effect.
2) Range is Self until ML86 (then Touch).
3) Only with a casting Spell Misfire is there a possibility for special complications as discussed in the spell; and only if the subject were currently under the effects of another rejuvenation spell at the time of Misfire or if the subject had already received Balm at least once before at the time of Misfire.

If so, you'd roll for Spell Misfire for the caster as usual, and then also apply some added "Balm damage" to the spell's subject (either the caster, if self; or the target subject). Perhaps the complication would be a Shock Roll or Reversal of Effect.

4) ML91+ Bonus Effect would remove the added Misfire danger from current rejuvenation spell.
5) ML96+ Bonus Effect would remove the added Misfire danger from being subject to Balm more than once.
6) Rule number one as worded would now allow the Caster to do the cast-and-dispel approach to increase their ML.

Each casting of Balm on average lengthens life by 5.6 months, correct?, so we're only talking about a max number of about 5-6 years for a character with high Spirit (or AUR). You could tweak this to suit yourself (half Spirit SI; double Spirit SI, etc).

Walt McAtee
Kelestia Productions

If the first casting was a MS, your character will only age two months (per SP30), and the spell ends and they can recast it again practically immediately.
If the first casting is a CS, the character ages 3.65 days in one year, the spell ends on its anniversary (per SP30), and the character can recast it.
If the spell's Duration runs out, the residue is gone, just as much as if it is dispelled - dispelling means that the spell is no longer there, and it's the same if the spell runs out - it's no longer there.
If the character is paranoid about the residue lingering, they could dispel the effect just after its Duration expires anyway, then recast.
Or develop a level VIII version which starts at the OML without a residue.

Chésmè appeared as an NPC in Robin's on-line experimental campaign when she would have been something over 600 years old, so it's fairly clear to me that Robin intended the potential life extension to be considerable. Having said that, I can see several ways that you might apply the spell in any given game.

The "residue" might be an indefinite effect of the base spell if you want to make learning and maximising the use of the spell particularly punishing (that would make a Dispell VII ML of 81+ necessary for removal). A far more lenient approach would say that, as suggested by Alex, the residue only subsists while the spell is taking effect (ie. for the Duration of the spell). ML 91+ would then allow other rejuvenation or similar effects to be applied simultaneously with Balm of Chésmè but with somewhat unpredictable combined consequences (as per the spell description). ML 96+ would them allow Balm of Chésmè itself to be cast multiple times simultaneously to get very low rates of ageing indeed without risking the variable combinatory effects listed under ML 91+. The limitation on such repeated use would be the repeated risk of misfire, which would eventually become very great indeed - ML 96+ may well remove the risk that a misfire will result in misfire of all existing, simultaneous rejuvenation spells on the subject, which would very possibly be fatal!

I'm sticking with the RAW - if the spell's gone, there's no dangerous residue.

This is one of those spells that a Shek-Pvar would probably cast on themselves repeatedly, and set aside a day to cast it. If a mage takes the time to cast the spell slowly, increasing their EML with a Focus and in a Sanctum, and probably use every trick to increase the EML further such as using IV level Cant and IV level gestures (remember, they're doing all of this in the privacy of their own sanctum), the misfire risk would be greatly reduced, and even the chance of a CS increased.

Besides, it could be good practice for any Shek-Pvar to cast their highest Dispell every few days or so, to cleanse oneself of accumulated negative energies which might have built up from spells cast on oneself to enhance performance, etc.

The only other thing I might think of that could help would be some sort of enchantment which stores this spell, one which can be refined and improved upon over time to accumulate the effects of increased EML, CS, and so on. But you'd probably have to be a Grey Mage to develop the necessary Neutral spells for this.

Or find a grimoire which contains research already done by another (by which, read "the GM").

Chesme IS a Grey Mage and we don't know exactly when she crafted this spell. It's possible that she had several assists from supporting spells, items, and places while practicing. I imagine her as very well prepared with a great many resources.

I'm not at all sure that the rules as written are that unambiguous. They talk about a "residue" that is left by the spell - residues are things left over after the basic thing is done, so I can quite see a reading that posits these as indefinite or even permanent features of the target's pattern or aura. The only thing said about them is that dispelling the spell - which would have to be done during the duration - removes the residue.

Of course, you could assume that the "residue" dissipates after the spell duration comes to an end, but I see no prima facie case that this should be mandatory. Failing that, once the spell duration ends, the only real option would seem to be developing a spell that specifically removes this "stain" on your aura; Chésmè herself may well have developed such a spell.

Dispelling Balm removes the residue. Dispell does not suspend the spell: it eradicates it. Which means that once the Dispell works, the spell is gone, and taken the residue with it. If the spell is not present, there is no residue - which means it's okay to let the Balm lapse, and cease to be, and take he residue with it - then immediately recast Balm.

The whole residue issue is just to prevent some newbie Fyvrian player character immediately reaching for this spell and casting it a couple of dozen times all at once. I believe it''s a mechanism to force the Shek-Pvar character to have one, and only one, instance of Balm running at any one time - at least, until they gain proficiency in casting the spell.

I don't think NRC ever intended for the residue to be a thing that persists after the spell ends, though if you do feel it should be interpreted that way, nothing stops you from learning Dispell anyway, and casting it immediately the Balm spell lapses to ensure that the residue is gone, then recasting Balm.

I might recommend storing Balm in some enchantment to cast instantly, once the prior Balm elapses. I might similarly recommend putting Dispell into a focus or some such artefact - it can be useful to have a staff enchanted to dispell unwanted magic, full stop.
I'm tempted to come up with a low level Neutral spell, Scrub, which does one small thing, but it does it well - gets rid of lingering spell residues on objects, animals, places, and people.

Dispelling Balm removes the residue. Dispell does not suspend the spell: it eradicates it. Which means that once the Dispell works, the spell is gone, and taken the residue with it.

This much is perfectly true. Note that the slowed ageing that has already happened is not reversed (except maybe on a CF).

If the spell is not present, there is no residue - which means it's okay to let the Balm lapse, and cease to be, and take he residue with it - then immediately recast Balm.

But this is pure supposition. Nowhere does it say this, and a general reading of the rules (with instances like Periapt) strongly suggest that it is not a sound one. You are free to rule this locally, of course, but I don't think it's the primary option supported by the rules as written.

The whole residue issue is just to prevent some newbie Fyvrian player character immediately reaching for this spell and casting it a couple of dozen times all at once. I believe it''s a mechanism to force the Shek-Pvar character to have one, and only one, instance of Balm running at any one time - at least, until they gain proficiency in casting the spell.

There is really not that much benefit to running multiple Balms at the same time - you can stack MSs to quarter or eighth etc. your ageing rate, but cast-dispelling until you get CS will be much more effective for similar risk. The supposition that this is what "the whole residue issue" is for seems to be quite a stretch, to me.

I don't think NRC ever intended for the residue to be a thing that persists after the spell ends, though if you do feel it should be interpreted that way, nothing stops you from learning Dispell anyway, and casting it immediately the Balm spell lapses to ensure that the residue is gone, then recasting Balm.

What Robin might or might not have intended is, unless you have extraordinary resources, pure conjecture - but Dispell as a remedy is of course possible and explicitly designed to be so.

I might recommend storing Balm in some enchantment to cast instantly, once the prior Balm elapses. I might similarly recommend putting Dispell into a focus or some such artefact - it can be useful to have a staff enchanted to dispell unwanted magic, full stop.

You seem to be confused as to how foci work, but a Major Artifact with both Focus and Vessel of Íladan or Store with Dispell in it would indeed be useful, if demanding to create (it would need an Ego or Will of 6 or 7, depending on whether you used Store or Vessel of Íladan).

In addition to which, casting Maintain on the Balm before its exhaustion would be somewhat easier and simpler (although you might want to Dispell and recast to get rid of residue which also affects "other rejuvenation spells", arguably like Succour Of Blezíl, Regrowth of Evâla, Regenesis or even Balm of Grésan. If the residue was 'intended for' anything while the spell is active it seems to me that this is a far more likely factor than multiple castings simultaneously of Balm of Chésmè.

I'm tempted to come up with a low level Neutral spell, Scrub, which does one small thing, but it does it well - gets rid of lingering spell residues on objects, animals, places, and people.

Yep, that could work. One specific to Balm of Chésmè might even be just level II or III.