1. Yes they would mare the surounding area. That is why you need to strip the paint and primer well away from where the filler work is. Leave at least five or six inches past where the filler will be bare metal. By the way, they don't always end like they begin. Very often, you find a need to strip more paint off. The skim coat should simply cover all of the previous coats.
2. Man, that is a tough one and would take an entire "Basics of Basics" to explain. In a nut shell you need to work a small area at a time. You sort of work many small area independly while looking at the big picture at all times.
3. Plastic filler should only go over bare metal or properly applied epoxy primer. Polyester putties can be applied over sanded paint with no ill effects.
4. I remember doing that when I first started. To tell you the truth, it adds a LOT of work. Better yet, just guide coat it with a very light mist of black lac paint in a rattle can. There are rattle cans labeled for Guide coat now a days, get one. You can see exactly what it will look like by blocking it (and hand sanding around those wheel wells and such). The guide coat will sand off very fast and leave the bondo. You will get a hang of it pretty fast. By the way, using masking tape to guide you on straight lines (for that matter I use it on curved lines too) is a big help while sanding.
5. Yes, more coats are needed. How many, only you can know. It really depends on how much surfacing your work needs.
