A lot of the fun I have is the refurbish of a stove, (just dont tell my wife I think it's fun as I tell her it's a lot of work).
1) I am using Rutlands Gasket and Stove Cement in the tube that goes in the caulking gun and it is pretty thick so maybe it is the heavy type
2) As for brands I don't think there is actually a lot of them to choose from, case in point I am a fan of "Williams Stove Polish" and I just noticed on a tube this week it said Rutland. Just try to match the temp as high as you can get it, the above i use is 2k if I remember correctly.
3) I use two layers myself but what I do is a bead on each of the parts with fiberglass gasket between them. (Norification)
4) Snug the hardware and after the first couple fires heat the cast and it cools go back and recheck.
5) Lining a fire pot requires currying unless it sits over an extended period like the summer, but the seams I have not seen a need to cure it dries pretty quickly. A small fire to cure the polish or paint is a good idea and keep the windows open if done inside.
Hope this helps and don't forget to replace the old hardware (nuts, bolts, washers)