I was able to do this in under 10 minutes. Very good write up.
I noticed that I lost all of my pictures and videos while completing this procedure. My text messages and other data remained, but my pics and videos were wiped out.
Did you check to make sure the antennae did not get disconnected? It is easy to disconnect, so you have to make sure to check that it snaps into place. The connector is the gold piece connected by a black wire.
Yes, there did seem to be a lot of glue holding my battery in. At first I was afraid of breaking it, but after reading up on the subject I've learned that it is okay if the battery gets a little deformed. Due to the nature of the battery it will still work if slightly deformed.
I had the same exact thing happen to me. I couldn't see where this piece came from either. Just like you, I gave up and reassembled my phone. It works just fine. I still held on to the part just in case, but I don't expect any problems.