Since a solvent is anything that will dissolve another substance, I'm pretty sure he does want some kind of solvent. Not all solvents are very harsh, in fact you drink one every day...water is a very effective solvent...many things dissolve in it.
For stickers that are very stubborn I have used eucalyptus oil with great results, but I admit I've never tried it on an acrylic crystal. It does leave a very nice smell behind though!
I would not worry too much about what you use - just don't go crazy and if something "fogs" I'm sure some polywatch will clean it up without too much trouble. I have wiped down crystals with 99% alcohol without any problems.
Cheers, Al
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