Is the tail panel welded to other parts? What fills the seam between the rear quarter and the tail panel? How big of a job is it to replace the tail panel?

Put it this way if you can pound out and flatten the panel and get away with a 1/2 gallon of BONDO do it. The tail panel is attached to 3 full panels; the 2 rear quarters with a specific contours, the rear valance plus the trunk pan which has bumper support brackets welled to the uni-body frame getting the trunk lid to lie flat with the latch mechanism is another thing.

By removing the rear panel you are litterly removing all the strength and support to hold the rear end of the car together. I would suggest you have an experienced body man look at it. If he recommends a new panel be prepared to do lots of work or shell out for shop time and labor. Just about all the shop tools are required in this case.

It isnt that hard to do, I did mine and it was fairly straight forward to replace.
I would suggest if you choose to do it then go purchase some spot weld cutters, it will go a long way in helping you get the panel off.
Make sure you keep the car level before and during the replacement.
Just take you time and it will go just fine, and to answer your question about the welds, yes it is spot welded in a lot of places.