I think you're somewhat right on 15. It's not totally bike friendly, although it certainly is doable. The stretch from Rt7 to Rt66 I'd try not to hit at morning or evening rush though would be my only caveat.
Much better than the north side of 15 toward the MD line. I agree that 7100 would detour you, and although it has a bike path along side it's a bit chopped up by cross streets and a lot of short rollers since it doesn't follow the road contour as much as the hills alongside.