We took a taxi from Phnom Penh to Sihanoukville and it took approx 4 hours Can't remember the exact price but it wasn't outrageous. Our accomodation booked it for us so we didn't have to do any bargaining. It was definately worth it as the buses obviously only run at certain times whereas with a taxi you just book it for whenever you want This is especially good on a weekend when the bus services are less frequent and often get booked up way in advance. Also the buses were packed with locals carrying all sorts of goods and you can often end up squashed in a tiny seat with no air con and a local woman sat next to you carrying loads of bags.

Definitely go with the bus and if you need help, book through your hostel - usually they can bargain for a much better rate for you if you agree to pay a small storage fee at their premise for your non-beach items. (PS - I definitely recommend leaving behind the big pack, travel in Sihanoukville is mostly on motos and not on great roads...)!

I usually prefer to take the bus. It takes 4 hrs and is comfortable and convenient with no bargaining. Shared cars are more complicated as the driver usually won't leave until the car is packed or the passengers agree to pay for empty seats and they will stop whenever they feel like it. If you don't speak Khmer, they will probably try to overcharge you as well.

no idea, not so much. Some coasts are nicer then other parts.