I think you know everything. I've tried mixing road and MTB FDs a few times, mostly older stuff 9-10spd. Never really worked.
Pull ratio difference: depending on your setup the shifter can click too early and not move the FD enough, or it might not be able to click because the FD will hit the limit screws first. Neither of them is ideal, both of them will cause terrible or non existent shifting.
All FDs are designed for certain size chainring. Let's say you have a road FD that was designed for a 50-52T chainring and you try to run it with mountain crankset that has a much smaller chainrings. The angle and the radius of the cage won't match the chainring. One part of the cage might be touching the chainring while the other part is miles away from it. Again, terrible or no shifting.
FDs and front shifting in general seems pretty simple but it can be quite finnicky. I've run into terrible shifting with much smaller mismatches that I thought would never make a difference. Back in the friction and no shift ramp era it was a different story. 😃
Of course we are on the internet so you will see people saying "Oh i've been running a Campagnolo FD from the 70s with SRAM Etap shifters and quadrupole chainrings, works great!"
Someone in another thread said they emailed SJS Cycles and they've said a Shiftmate 7 installed with the cable entering the small pulley groove should work for a double MTB front derailleur as well as a triple. I can neither confirm or deny that.
Haha, I know hardly anything, but thanks for the compliment. Thanks for the tip on the ShiftMate 7. My plan right now is to purchase one for cheap online, and see if I can rig it up. Next would be to try and mix the 5800 shifters with the SRAM FD that's currently on there. I would sacrifice some performance if it just "worked," but everything has to be reasonable. The SRAM GX(?) FD that's on there right now works great with the SRAM Red shifters, but the Exact Actuation took care of that aspect. If we have the cable pull info for every Shimano road shifter as well as the road and mtn FD's, it'd make this search much easier!