I mean, yeah, the body would look different when being compressed by the clothes, but in most cases, the only notable difference would be the boobs. Most of the time, sure, there'd be some indents from waistbands and such, but they wouldn't be noticeable enough that you'd have to include them, people probably wouldn't notice that fine of a detail. The boobs getting supported by a bra or let hang free would be more notable of a difference on characters with big tits, but technically speaking, it wouldn't increase the number of different art variants you'd have to do. If you feel capable of drawing boobs as if they were being supported by clothes without them looking weird, then I don't see any problems with that. If anything, it makes it easier on characters with cleavage/other partially-exposed boobs, since you can reuse the part where they're shown. And it'll only actually come up if the character isn't wearing clothing that is either too loose to restrict the boobs, or is skin-tight enough that it's basically just the boobs anyways, and I notice you like to do a good amount of the former.
I do see what you mean about the fluids, now that you mention it, since there's a minor point of "oh, well, if there's actually clothing being worn over this, so there shouldn't be sweat drops on the skin here", but I doubt it'll be too noticeable. Even less noticeable than the non-boob-skin-indentations.
I don't mean to say that it's all about hiding it, but there's a level of plausible deniability. Like, looking at IRL exhibitionism kink, the risk of being caught is part of the thrill, so in this context, no, they wouldn't just wear baggy clothes to hide it. Honestly... I think you could do both, though. Make the nude-hypnosis art now, then later, you can come back and add the xray-clothes-disabling, and you won't have as much art to make. And, the idea of releasing the hypnosis in public seems too hot to pass up, especially with the general porn-logic "nudity is legal, but still embarrassing" reason to explain away why they wouldn't just cover themselves immediately. Although, I do think it'd be better for there to be a "underwear only" in-between stage even for that, so it doesn't go straight from fully clothed to nudity (well, not really "straight" there, since it takes a while, but you get what I mean). And hell, if you were feeling extra ambitious, you could even have alternate costumes that you could hypnotize the patrons into wearing.
Honestly, gameyness is good, that's what elevates a neat little sandbox into an actual fun experience. So, yeah, go for it when it comes to the timer, that's a good idea when it comes to that. As long as you make sure it's not too tight that it's repetitive. Although, I think a better system would be "Incorporating the personal items into the voodoo doll makes me able to keep the magical link open for longer at a time", since putting the items in the doll feels more voodoo-y to me, and it'd make it make more sense in the case where you fail to reach the objective in time and you have to try again the next day. Unless they're coming in the next day with the same exact type of item to burn along with their letter, in which case, that could work.