I made good progress with carefully prying the left edge of the battery up with the sponger. The battery did finally come free but it was pretty beat up. I was fortunate but can see how the pry method could possible rupture the battery. I should have read your comment about using a length of string. Oddly, I use the same technique to cut PVC conduit. A length of heavy string sawn back and forth will melt though the PVC slowly without damaging the cables inside.
“Rice in a Ziplock bag” worked perfectly. I was skeptical at first that the heat wouldn’t concentrate enough on the adhesive to soften it. I just rested the battery side on the bag of rice (1/2 cup basmati left over from a 20# bag of Y2K supplies :) and slowly lifted up on the left side of the battery CAREFULLY leveraging with the sponger over the left edge of the case (all the other sides of the battery have electronics that don’t need to be crushed. It took about 4 mins to pry my battery free.
This worked extremely well. Thank you! I used the tail of the zip tie as a nub to help hold down the phone while doing what you described. The left side of my phone opened easily but I had to use a combination of spunger and X-acto blade to free the end and right side.
Once opened I placed the same plastic sponger just under the screen from one side to the other to hold it up to access home button cable.