it's fine to use one side of the board, just pan everything hard to that side. for mono, you would indeed center all the pans and hit that button.
as for headroom out of the A-B outs, it's OK like it is, but down the road you can get the upgrade chip installed that will kick it up to true +4 balanced out. i did that to mine, one each for A and B out, and it was a marked improvement.
that hosa cable is an insert cable, which is what you don't want for this. effects get put on post-fader auxes, so one effect can work on any and all channels, and so that the entire channel's signal isn't forced through the effect (which can sometimes degrade the sound).
yes, people will insert another special reverb just for the snare drum or whatever, but that's for down the road. inserts are for things like compressors, EQs or gates, things that affect the actual volume.
since your amps apparently have their own DSP and stuff including crossovers, you can send to the subs amp either daisy-chained with the signal the tops amp is getting (A to one, B to the other, A to an EQ, and if the EQ has two parallel outs, one to each amp, or even into one amp and out the other signal input (not speaker out!!) to the other amp.
the amps will both get the same signal, but one will only do lows while the other only does highs.
you'd then just adjust the volume of the two amps so that the highs and lows are balanced; then, you'd use your high-pass buttons on the channels to keep whatever you want out of the subs.
for aux-fed, just use the main out to tops and a post-fader aux out to the subs. (trouble here is that turning the main fader up or down for overall volume throws off the balance of tops to subs.)
i suppose you could also use the left/right outs as tops/subs; use (say) left for tops, and pan everything hard left. then, just bring back towards center what you want to be in the subs, too. (remember, you don't want anything only in the subs, that's useless.)
this way, moving the two main faders at the same time doesn't throw off the balance of tops to subs.
i like mono with a normal crossover on my mixwiz (same as yours) because i'm using all 6 auxes (4 in-ear mixes, delay and reverb), and because i like to pan vocals on one side and everything else on the other and insert a compressor on that side's main insert for all the vocals.
this also makes it easy to quickly turn the vocals up and everything else down if need be.