Yeah, magnesium is somewhat of a pain killer, but in a muscle relaxant sort of way. It also acts as a NMDA antagonist in the brain, which potentiates Kratom (and opiates/opioids, like OC) and helps with tolerance.
Diphenhydramine (Benadryl) does indeed also potentiate OC (and Kratom), but has a more sedative and hypnotic character.
I would recommend against using OC, if possible, because it is addictive as hell and a dirty man-made chemical. Of course, if you need it, you need it and you should always listen to your doctor first, but if you are, in every possible way, able to exchange it for Kratom, there should really be no reason not to.
Glad I could be of some assistance.. I know a thing or two, but you're giving me too much credit!
Sorry if I'm getting a bit OT, mods...