first of all, o_o do NOT shove the re-root tool in first, you might break a lot of needles doing that. :p id suggest getting a needle, slightly bigger then the needle in the tool, make your hole with that first and pull it out right before your about to put the hair in, the plastic will shrink quickly.
im not sure what to do with the hard back side of the cam heads, as i havent rooted one yet, you could try rooting around it, if you plan on keeping the hair long and thick enough it might cover it pretty easy, otherwise I hope someone else can help you there.
as for the hair line, you could use one of the wigs as a template perhaps, some of the wigs iv found have a pretty high hairline, for instance my witch/cat wig is just weird, but my skeleton wig is great. or you could just look at your other dolls if you have any and get an idea from them. you might want to sketch out with a pencil or something that can be washed off easy without staining your dolls head.
im not expert, and these are just a few things i planned on trying when i customize my first cam, so maybe they will help you as well. im sure people with more experience will comment soon