Sounds like he wasn't much of a Genius. Technically he can't give you that advice, and if he does he is liable for it. (Or Apple is anyways, you can't do anything to him but you can to the company and they can to him.) If he made any disclaimer this wouldn't apply, but if he did you could try fighting it. Most likely you wouldn't win, but legally you have a case.

As far as disassembling, if the residue is simply remaining liquid, then yes you can clean it off. But if there's any damage already done to the display or the glass you may not be able to repair it or would have to replace the part. I wouldn't be able to answer without actually seeing the phone with the screen off as to whether the display is damaged or not.

If it seems like liquid is just moving around in there, I'd say your fine. But if it seems like the picture is changing, as if the electronics or the display is changing like when you press on an older LCD screen, then you may have a problem that cleaning won't fix.

There's no harm in trying if it's something your comfortable doing. If you're not comfortable doing it, I'd recommend finding someone who is. You're not going to do more damage if you're able to put everything back the way it came, as long as you don't spread any of the acetone moisture into the rest of the phone.