[MikeCStig]

1. There's probably a hole in the floor somewhere. Those vans do rust on the underside quite easily.
2. This sounds incredibly dangerous... don't use the cruise control until it's checked and repaired successfully.
3. You may very well be able to get the bolts, but I can't guarantee they'll line right up. You may have to reallign the brackets.
4. I would think that there are threaded holes accessible from inside the van, they may, however, be stripped in which case you'll have to re tap the threads and use oversized bolts.
5. The mechanism may be jammed or broken. Since the rear seat is detached anyway, you could pull it out, take it apart and figure out how it works, what's wrong with it, and maybe fix it
6. you may have to put new elastics in, and some panels may have to come off, unless there's material strong enough to hold the elastics in place with rivets.
7. it's likely to be mechanical. That sound you're hearing is the motor trying to do its job. You'll have to take the inner door panel off, get somebody to push the button while you watch what the mechanism does, it shouldn't.be too hard to figure out
8. the cargo doors (rear doors) are often not a part of the power lock system. That's probably the case with you van as well