Derailluer-like chain tensioners are for retro-builds. Don't plan a new build using one; get sliding D.O., horizontal D.O. or eccentric B.B. Direct chain-routes run cleaner and quieter.
How do you run a rack with seatstay mounted disc calipers?
Again, don't use if you can avoid. The "solution" is a disc rack with 1" of spacer on each side, placing the panniers 2" further apart, increasing air resistance and reducing space when riding through a gap.