I have been making leather armor for some time now. To answer one of you first questions, Yes, it is possible to rivet you pieces and wetform the leather after. That is how I've been doing it for years. For the form, you could either buy a plastic manaquin (they come very cheap if you buy it used) but be carful to pick one that correspond to you bodyshape. Or you could try to make one out of plaster using your own body.
When trying to make a pattern for armor, I personnaly don't use cardboard, because of all the curves, paper and cardbords are too rigid to give you an exact idea of the shape you will need. I prefer to use thick fabric, I place it on me and draw the design right on me. That way I loose less leather.
Hope this was usefull for you, and Have fun!