I don’t have any tutorial links. Yes, mesh or surface deform will let you pose a low poly and have the high poly follow (and has better performance than an armature in Blender, in my experience, even though that seems crazy to me.)
I think that’s too high poly for a low poly. You’ve got to be looking at 300k+ verts at least for the whole body + face. Probably more. My personal rule for a low poly is under 100k, and that’s already stretching it really, really hard. A more reasonable rule is under 25k verts.
Something is creating that line. If you want me to tell you which bone, I’d need a file. Judging weights by colors is rarely a good idea (because weights <1/255 can still have a strong effect, and there’s no possible way to see that in colors, even if Blender didn’t choose colors that are really hard to see to begin with. Maybe that last part is just me, but I can’t see the difference between 0/255 weights and 1/255 weights, and I suspect that if I wanted to tune my monitor to see it, I’d be seeing banding in all of my renders.)