Hi! The retopo is quite a difficult task, but still satisfying. Here is my troll all done by hand 
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I wanted to smooth out the faces, but the vertices stick to the hp of the model and as I move them they jump by quite a distance. Do you know any way to fix this?
Because of this, I can’t do a straight elbow and a few other parts
I think they look fine.
Fix problems, on encounter them while posing the Troll using a armature.
Looks good to me. So much better than auto retoppo.
Everything looks great to me. It’s actually better that the elbow isn’t straight. During the rigging process (if doing it manually) you will need to define which way the arm will bend and it could make it go backwards (not difficult to correct, but why make extra steps). I look forward to seeing more progress in the future. 
Also, to avoid the vertex from jumping everywhere, auto-merge vertices may be on.
