1) The foam is on the outside of the wall assembly, not the inside. It provides the WRB (weather resistive barrier) - and there is a peel and stick membrane between it and the OSB for multiple reasons. You can see a discussion of this technique here:
http://www.forestryforum.com/board/i...?topic=57351.0
It was also detailed in a Fine Homebuilding article several years ago.
2) I nailed every 6" around the perimeter of the OSB sheets, and on the inside every 12". I don't recall where I found the fastening schedule, but it should have been something current.
You can see progress photos here:
https://www.flickr.com/photos/68046081@N00/
I will certainly prime all edges, but don't know if there would be any benefit to painting the back side of the siding. I do not expect much moisture behind the siding in the form of rain or snow/ice, however since the back of it will be vented, any water should evaporate.