Insulation, Radiant and Vapor Barriers - foam under rafters for cathedral ceiling?

I am remodeling my attic in an older home. I only have 2x6 rough cut rafters up there now. The roof is low slope, about 3/12. The lowest points of the roof with unfinished attic space are ventalated. The roof is covered with a torch down modified bitman rubber material, not shingles. There are no vents at the peak.

I propose to use R-21 insulation that is designed for 2x6 walls. Then place an inch or two of foam under neath the rafters to get an overall R-31.

My questions are:
1) Can the sheetrock be screwed to the 2x6s with long screws through the foam?
2) Should I install faced or unfaced R21 insulation. I think it should be unfaced and let the foam act a a vapor barrier.
3) Because my house has a low slope roof, I think I do not have to ventalate the finished attic. Sound correct? My assumption is that ice dams will happen on my roof, but because it is a completely sealed roof, rather than shingles, everything will be fine.

Additional info / thoughts?

I just read that foam board is not a very good vapor barrier [1.4 perms vs. most vapor barriers under 1 perm]. Thus, I worry that I'll get moisture in the FG insulation without a plastic barrier over the foam board. I also suspect I can't use faced FG in between the rafters above the foam board because it will trap the moisture in between the two layers. Because faced is preferable for the istallation, can I install faced FG in between the rafters, but literally slash the facing after its stapled up to prevent moister buildup in between the FG facing and foam board.

Finally, how does drilling [to install sheetrock] through the foam board and any plastic vapor barrier affect the vapor barriers performance. I assume you just don't worry about that.