1 yes , ti depends in your build, gear and strategy
2 NO they have great lp and defense that make the difference in battles. if you have a good gear it will be really difficult to kill you in battles

3 my DW without buffs at lv43 have like 10k if im not wrong, now lv51 he have like 13k lp with buffs like 16k

4 SK can play in PvE and PvP it all depends in your build but DW only good in PvE (if I'm not wrong in the only kind of PvP they are good are for dojo wars)
hope it helps you