If you want to maximize the R-value of the R13 (or any insulation for
that matter) then you should use expanding foam to seal any
penetrations through the exterior wall and/or each cavity. This will
create dead air space in each cavity and allows the insulation to be
most effective. You can use the low expansion stuff.
Check the price of the R15 vs the R13. R15 tends to be very pricey and
if you're going to use Tuff R then I don't think you'll need it. You
can simply increase the thickness of the product. I just would go any
thicker than 1.5" because then you'll have a hard time keeping the
drywall flat.
Any way to attach the foam to the stud temporarily will be fine since
it will be screwed tight once you apply the drywall over it will also
fasten the foam. You could probably nail it but make sure the nail head
doesn't stick out since it will also interfere with the drywall.

Good luck.