In case anyone is wondering, I was able to get the two small back covers to reassemble without using new adhesive. I didn’t have any on hand so I just used a heat gun to warm the adhesive that was left over and firmly held them in place. So far it seems to be holding well.
If your new trackpad won't fit into place, loosen the screw at the bottom, center of the case (you can see it in the step 36 pic). Don't forget to tighten it back up. This screw affects the amount of play and click in the mouse, so fine tune it until the pad clicks to your liking.
This adhesive was extremely strong for me and took quite a while to soften up (longer than batteries). I ended up using my wife's blow dryer after the heat pad struggled with it. Just take your time.
My adhesive didn't have enough tack to hold the battery in. I just reassembled without it and can't hear the batteries moving, but I have the rubber grommets and not screws as MarkB mentioned. Fo future repairers, I'd recommend buying some nonpermanent two sided tape just so you have the option to fasten it in.