I've cut a few top trims off of tanks where the "silicone blotcher" has been very generous. The nice part is that, if you have a new trim to install, you can carefully break off pieces of the old one as you go. I have to say that, replacing the trim on a tank that is up and running sounds like a challenge to me. You certainly don't want pieces of plastic or adhesive dropping into the tank.
If the plan is to glue on an additional piece of plastic, I'd suggest removing enough water so that the crack begins to close on its own. Again. you'll have to work carefully to make sure nothing gets in the tank, but you need to clean the surfaces you gluing very well, probably scuff them up with some sandpaper, and I'd suggest using an epoxy product, rather than a superglue. Better to get everything right the first time, than to have to do it again. Good luck!