Mix the 24/48 sources to 32/48 first, then dither to 16/48. Some apps allow you to accomplish both the mixing and dithering in one step (in the proper order), other apps require two steps.
I've mixed a mono soundboard with a pair of mics before. Worked quite nicely. There's a good chance the board feed was mono, anyway (though perhaps dual mono, i.e. same content both L and R channels). Nothing wrong with mixing mono sbd with the mics, especially if your ears prefer it.