You won't be able to find a solvent that will eat cured silicone.
Easiest method I have found is to use the small router bit for a dremel and to just melt/grind the plastic off of the top only. The bit doesn't affect the glass edge. You'll have an inner and an outer piece, and can easily slice off the silicone to free them.
I've tried using a razor plane to shave down the plastic, and that was just almost impossible. I've tried whittling it down with a xacto knife and that was also too difficult.
If you've completely cut all the silicone the piece should just slide off with no risk of breaking the glass.
On the other hand, I've spent alot of time and money going for a rimless 2.5g, and I could have bought an ADA / Do!Aqua tank with twice as thick glass for the same amount I spent... You can get an actual rimless tank for $60, the glass will be thicker, and you'll sleep better at night, knowing it's not going to be as fragile..
Might try just melting the plastic with heat off the top of the rim... Line a cookie sheet with aluminum foil, put the tank rim on it, put this on the stove and slowly crank up the heat until the plastic starts melting and you get down to the glass. Still probably have to cut the silicone..