I don't know about CoH2 but when we played 3v3 or 4v4 in CoH1 with a wildly mixed skill team, there was only one real problem in that the AI cheats. I'm not talking about the resource bonus or anything because the "hard" AI that replaces players is really too stupid to do much with that.
The problem was that the AI would use pios and other small units combined with its knowledge about player unit positions to sneak through lines and capture cutoff points on non-Scheldt-style maps.
For some players this distraction is a lot worse than just having an increased amount of units thrown directly into their killing zones. I can only guess this is the same "problem" in CoH2.
The answer is to get better at the game but until then, actually having light vehicles (and using them to hunt those infiltrators, US ones can even recapture) instead of just static units and artillery should help. If you are higher skilled than your team mates, picking up their slack in this department should be your job. They probably won't learn much but at least that way they can enjoy the game without being stressed out about sneaky units.
The game simply can't decide if it should replace a player with easy, normal, hard or expert AI. In any case this would probably just create more problem. Hard AIs really aren't that hard to beat. I absolutely believe their game impact can be higher than some players' but that ultimately isn't an AI problem.
i dont think sneaking behind enemy lines is really a "cheating" thing, its very easy to do by pure accident. Just stroll anywhere away from the heavy fighting.
The main difference is that most players play with a trench warfare mentality. they want to dig in, set up defenses and maintain the land they get. then use arty to soften up the enemy and push up with whatever tanks they can build and push the line foward.
The ai plays more neutrally, it doesnt dig in, its forever in "skirmish" mode where it just wants to poke at the enemy and distract them, run off and cap a point somewhere else. It sends out small units all over the map, and some of them are going to find the holes in your line. Bassically its playing 1v1. Unforuntately 1v1 tactics dont always work well in 4v4.