Okay. First of all I am no expert in realism at all, so anything I say should be taken with a grain of beer...I mean salt.

1. Yup all the characters that are sold have perfect bodies and what not (because they sell). Except few. As for skin shaders...I don't know BagginsBill knows more than me (times 100) but realskin shader seems to be popular among Poser users. Made by Face_Off. You might want to check that out. Allso check out threads by BagginsBill in here or at RDNA.
   Yes. Lighting is the most important element in Poser renders IMO. The reason you can't re-create the image you see in promos is because of the lights.  Honestly, I couldn't re-create the promo renders myself without the exact same ligthing.

2. Well lighting in Poser probably can dublicate real world lighting quite well. ( I think) But it takes ...I dunno years of research. Maybe? Use Image based lighting for starters and compare that with standard light and you'll see the difference. After you see the difference you can work out the flaws and see what works and what doesn't.

3. With P7 you have morphbrush. Look into that. It can really help you with pokethrough. Allso phtoshop is your friend or PSP9 :D Then there are magnets. After that it's 3rd party programs. Personally I use paintshop pro.

4. Well try Kozaburos hair. http://digitalbabes2.com/
   Dynamic hair allways looks bad to me. Might be a better idea to handpaint that. Or use prophair.

P.S. Pardon for bad English and hopefully the smarter Poser-people will give you a better answers.

-Morbo will now introduce the candidates - Puny Human Number One, Puny Human Number Two, and Morbo's good friend Richard Nixon.
-Life can be hilariously cruel