If these software controllers allow you to completely customize which faders do what, then there is one other option that doesn't require one to use a separate instance of SAC Remote for each band member.
What I have done in the past is to create one mixer in SAC that is for my In Ear mixes. And I set up submasters to mixdown "stems" for each band member. For example, in one show I had a 4 piece band. I set up 12 submasters in SAC, three for each band member. So they each had one controller for "me", one for "the rest of the band" and one for Vocals. Now you could simplify that even more if you like, and just do a "me" and "them" control for each person.
Now when I did this, I was actually hooking up physical surfaces, in this case BCR2000s. I had 2 surfaces and two band members would share one surface.
In order to make it so one person didn't mess up someone else's mix, I put the 12 submasters into the first slots on the F-Mixer. I then programmed the surfaces so that only the knobs I wanted would do something and each surface only had access to the 6 knobs that needed to be on that surface. I think I might have set up a master volume control for each output as well, I don't remember now.
Now on a BCR you can just program the unit in a way so any paticualar button or knob does "nothing" or it's "off" if you wish. If these apps are not able to do the same thing, you could just program them to send MIDI commands that would be ignored and it would have the same effect.
In this way you could potentially have 4 or more band members that each have only 2 functional knobs or faders. A "me" and "them" for example. And they are all addressing the same instance of SAC Remote and they all are addressing the same monitor mixer.
There is only one BIG CATCH with this method. And that is the "them" mix must be shared by all the band mates. So for example, in my set up we had 2 keyboards, guitar and drums. But the levels of the drums in comparison to the other instruments would have to be fixed for the guitar and 2 keyboard players. For example, Key 2 couldn't have more guitar than Key 1. That is a limitation that the band had to agree on and in my case it worked just fine. But it might be a non starting for others.
You could potentially play some games with using Aux Sends on that same monitor mixer as well, assuming you're not already using them for effects in the monitors. But that would make the routing even more complex and convoluted.