OK, answered my own question on the lumbar support. Basically:
1. Pop the plastic seatback loose by pulling gently along the top to ease out two nylon fasteners
2. Once the seatback is loose, pull upward to remove the seatback from it's mounting brackets at the bottom 1/3 of the seat.
3. ease a long, flathead screwdriver between the left side of the seat and the plastic seatback. Then gently pry up two fasteners along opposite sides of the lumbar adjustment lever post. It takes some doing, but I eventually got it to come loose. Once the lever is removed, you can pull the seatback free.
4. With the plastic seatback fully removed, I added cotton batting (I know...ghetto, but who's going to EVER see it) to the back of the seat along any areas where the plastic seatback was rubbing the seat frame. This made a pretty significant improvement of the number of knocks I was hearing.
There's still a noise deeper in the seat, but I'm not going to dismantle the entire thing to get at whatever the cause is. I'm feeling lucky that made it this far without breaking anything permanently
Total time for the repair...less than 1 hour