I'll take a stab at answering.
First, you need to define hard and soft top. By hard top, I assume you mean the removable or interchangable fiber glass top for soft top trucks.
The original aluminum hard tops for the station wagons are removable, but would take you an entire weekend to swap in the proper roll cage,etc, plus you need to gather all of the parts...$.
1) I'm almost certain their were no intended soft tops made with rear jumpseats.
2) Yes, the fiber glass version of the "hard top" is interchangable.
3) It's easy to sintall jumpseats. The only difficult part will be figuring out where to mount the seatbelts. In the station wagon, the seat belts bolt through the tub into the roll cage underneath the truck in the tire wells. It's stout.
4) Anything is easy to remove. I reverse bolted my jumpseats and added wingnuts from the top so that I could easily remove them. While the studs for the boly do stick up, having that added space is fantastic.
5) You can get the Yakima mounts for the nissan xterra roof bars, and mount a cargo basket. Or you can also get the full land rover safari roof rack from safety devices.
Not sure how many people you want to fit in those seats, or how comfortable you want them to be. But I have considered these fold out full size seats a solid alternative to the crappy jumpseats:
I say crappy jumpseats beacuse they are. The backing on them is almost cardboard, and if they get wet in your ST, I guarantee they will suck.