I know I’m super late but I didn’t have any experience with the keys until now. Once you get the back plastic shell off, you can break the melted plastic bits using wire clippers. Scissors might work but may be difficult to work with due to their length. Don’t worry about breaking the melted bits, your calculator will fit back together just fine without them.
From here you can follow [guide|11224|this guide starting at step 7|stepid=40130] for the TI-83 Plus since the calculator’s construction is roughly similar and it explains how to clean the contacts. I wouldn’t recommend removing the rubber membrane though.
Let me know if you have any other questions!