one shoe for two suits

I would pick mahogany or oxblood before the cognac, but any of the three should work fine.

I would probably go for a medium brown. I am not a fan of oxblood and if the brown gets too light it does not work with navy anymore.

Black should work.

Black would be a better option; however, if you want brown shoes then a medium brown would work well with both.

Light brown would be too light for navy, the same could be said about dark brown and light grey although it can work depending on the shade of grey.