Replacing screen, adhesive won't give, but screen moves visibly

I’m trying to replace the glass on an ipad air that I purchased used. The glass broke — nearly shattered — when it fell, in a case, about 18” onto a carpeted floor, so this is very likely not the original screen. I can’t get anything between the glass and the case, even after 2 min of the iopener or 15 min on a heating pad. There’s a spot on the left upper corner where the suction cup will adhere directly to the screen; I can see the screen bow out when I lift, but I get no gap even though the screen becomes visibly convex. Much more of this will probably break what’s left of the glass. Should I just let this happen and remove the pieces (screen is taped over), or is there a better way? The videos and pics I can find online all show what looks like a pretty easy release of the adhesive.