Re: How to retouch this hairline?
This kind of thing is annoying for everyone. First off you have to be careful not to chop it in a way that it looks like you chopped something. If the hair appears to be going that way, that's a problem. You can create a general hairline with something like the pen tool feathered by about 1 pixel, and remove all or most work that goes outside of it. That is generally better than a wavering line for the final hairline. It still won't solve the problem of what to do with that many little hairs.
Ultimately it obscures enough area that you can't just clone something over in a way that isn't noticeable, especially due to lighting differences. You can use individual strokes and clone lighten mode on a duplicate background or a layer set to lighten mode (use normal mode of the tool in this case). Retouch out any obvious pattern duplications that arise in this case. As you get toward the end you may have to copy over some amount of skin texture via a high pass filter or other methods to fill in any area that looks flat. The entire thing has to be shaded to match the appropriate lighting, and any highlights or subtle shading due to bony masses needs to be continued appropriately.
It's detailed work to get something that looks good, because you have to cover a large enough area at high density that any flaw in the work will be much more apparent.