That sounds plausible. I have seen first hand the effects of acetone on plastic. It basically dissolves part of it into a pool and would leave colored residue like that shown in the picture. Some kinds of plastic are vulnerable, others aren't, but I've never seen battery acid make cracks like that, so acetone is more likely.
I do have to mention though that there isn't actually a gap underneath the battery-side air vent. The nearest gap is the one on the opposite air vent, which you can see on this picture from iFixit:
So if the battery did leak, that would be the most likely escape route. Not sure how it would get under the stand though, since there is no gap for it to escape through there...
It would only fry if the liquid actually conducts electricity. Water on its own is not a very good electrical conductor, but tap water conducts electricity due to the minerals in it. I don't know about battery acid though.
The thing is, the battery would continue to work as long as there is charge left in it and the protection circuit doesn't trip. And the protection circuit would only trip under certain conditions such as overvoltage, undervoltage, overtemperature and overcharge. So I don't think that alone is reason enough to think it's not battery acid. It's hard to tell at this point but the acetone theory seems the most likely.
He just said I was right P: