1. A velomobile is most often a fully enclosed recumbent trike. I suppose it could have 4 wheels.
2. You clip in/out just like on a safety bike - you clip in with one foot, start pedaling, then clip in the other foot. When you stop, you unclip a foot to put down. Unless you're on a lowracer or a trike, in which case you don't need the foot down to balance.
3. Some recumbents are faster overall, some are slower. Most are slower climbing.
4. I'd consider 50-60 Kph a bike-friendly speed limit. Not that I can do it for long, but cars won't be overtaking you at dangerous speeds. Recumbents don't hop curbs, at least not in the up direction.
5. I recommend using Bentrideronline.com as a resource. I take it you're not in the U.S., so I don't know what your supply lines look like; but you're correct in noting that they aren't stocked by most shops. Maybe if we knew exactly where you were, we could point out a dealer.
6. Yes. Depending on the speed capability of the bent in question, you may be faster or slower, but you will tend to work just as hard.
7. Bent riders see the 'big picture' better than upright riders. For whatever the reason (and there are many theories,) being lower on a recumbent does NOT make you less visible to cars. In fact, most bent riders report that cars give them more respect and clearance than when riding uprights.
8. A trike doesn't take much more room on the road than a 2-wheeler, it just looks like it. Cars will push any bike over, if you let them. The trick is correct lane placement so they can't do it.