1. Yes you can graft mid sections. You can graft just about anything. Areoles, a half of a trich seedling sliced vertically in half, upside down scion, sideways scion, ect ect ect...
2. I only use humidity with young, small seedlings no matter what I'm grafting on. With older plants I don't though.
3. Yes you can do this. It's usually only done with very fast rooting stock though from what I've seen.
4. With any graft it's universal. Before the graft takes the scion usually goes soft/shrivels. When the graft starts to take the scion will firm back up and then new growth will start showing up.