Progress at last! Persistence paid off. After reading the very helpful hints above, I decided to give the trunk keyhole another try first. This time it worked! I recall from my 91' Toyota Celica's experience, that once the keyhole goes back to vertical, you can pop the trunk from the trunk release lever. This time I inserted the key, pushed it in with force, then turn it vertical. The key can't unlock the trunk, but I was able to give the trunk release lever a gentle pull, and it popped open!
Upon closer inspection from the inside of the trunk, I found the key lock was just retained in position by two metal tabs. So by pushing the key against the lock, I was able to give it enough clearance over those tabs and turn it back into position.
From inside the trunk, I found two lock switches for the back seat. They were both locked! so that makes sense the pull string couldn't pull the back seat down. I unlocked both of them, and only one side of the back seat would release. Since it's a one piece back seat, it still wouldn't open at all which ever way I tried... oh well.
I also found the two 10mm bolts securing the backseat hinges, on each side. They were a little hard to get to so I just left them along, for now.