Can I run stereo cabs but sum them to mono before they hit the output?
definitely.
i'll be doing an in-depth video regarding Stereo vs Mono sometime in the next month or so, but let me mention some ways to do this. this is not a comprehensive list, so someone else might come in here and say "chris is wrong, there are other ways to do this too"
first, it depends what you mean by "stereo" cabs. do you mean actual "stereo" where one is panned full left and the other full right? or do you just mean "two" cabs, for more tone choice?
if you mean a Stereo Cab block that has cabs panned full left and full right, the simple way is to choose Sum L+R for your Output in the I/O menu on the Axe Front Panel. this will make all jacks of the selected Output emit the sum of the Left and Right signals as a single Mono signal. this can cause phase cancellation, but that's the result of collapsing any true Stereo signal to Mono.
if you mean you are using a Stereo Cab block only to have 2 cabs affect the tone at the same time, simply pan both cabs to 0 or Center, creating a Mono Cab block.
curious - if you run Mono to FOH, are you running stereo elsewhere?
give me more details of your setup and i can suggest a few more useful things.