@The 42nd GeckoIt's more straight up mind control then actually relying on Miyu doing anything. It's like the Matrix except there isn't anyone to give you a red or blue pill (though Graf might do it for shits and giggles and give Miyu false hope of escaping her situation).
@floodtalonWell for starters, while Balrog and Graf are more or less in sync due to their mutual intentions (Kill everything), Balrog could use some extra demonic pressure to overpower Graf's control of his body. So basically, Henry needs to possess/sit in the pilot seat. And while Graf's own lesser Golden Eyes are helping give the metal colossus enough juice to fight, if Henry contributes with his real Silver Eyes, that'll give em just enough power to possibly get away from this situation in most of their pieces. Not to mention it'll give Henry far more control over Grafdakka. As for adding Miyu to the mix, again it's more of a bonus. She wouldn't really be in charge of anything, and it's more like we're siphoning her knowledge from her head and doing horrible, horrible things to her body while we're at it.
I should warn you however, Henry won't be able to teleport to Graf. Endian's Demensional Anchor is still active, so any sort of teleportation or movement beyond stepping one foot after the other is largely prohibited.