I think the "what if" is a legitimate exploration. The "galactic barrier electrocution" caused this psychological erosion in Gary Mitchell. We don't know enough about it, due to the way the script was written. Was it irreversible? It's hard to know. When he nearly fried himself in the brig force field, the silver glow of his eyes faded out and he seemed to be himself for a moment. Then he reverted back. By then it had already been a spread of days with him in this "God" condition. Later, when he was thoroughly shocked by Dr. Dehner, his eyes returned to normal. Then Kirk started pummeling him right away. Gary was dazed and basically getting his ass whipped, then began to fight back. I'd say it was the "human" Gary that fought back, because he was being attacked by Kirk, as a visceral reaction due to being so disoriented (his mind was "hostage" for quite a while)... rather than just a weakened version of the "Gary God."
We don't know what was going on with Gary and Elizabeth physiologically. Maybe their brains were being rewired and there wasn't anything they could do about it. Or... maybe there was a cure. Perhaps a means of "excising" the energy manifestation inside them. But I do believe it would've been possible. Dr. Dehner was reachable by Kirk, despite being in a God-like state. Maybe her personality manifested things differently, not as aggressive (or taking much longer to become oppressive and threatening).
Despite Gary having "God" powers, he was crushed by a huge boulder and killed. But... given his power, was that a legitimate ending? Certainly after falling into the grave hole, he could have flipped over on his back and used his power to keep the boulder from crushing him. Remember how easily he dispensed with that rock Kirk was going to smash against his head, and able to resist the enormous power of the phaser rifle. Well... allowing for that weakness, maybe he'd be unable to break high tensile strength metal, and Dr. Dehner could've used her power to manifest special handcuffs that Kirk could've shackled to his wrists and ankles. With Gary physically bound, Dehner could keep giving him mild electric shocks to subdue him. They go back to the Enterprise, and the doctor uses Dehner's power to shock Gary in just the right places for sufficient duration to eradicate the "evil energy." With that threat gone, the doctor and Mr. Spock come up with an electrical method of shocking Dehner to rid her of the power as well. Everyone gets cured, and the Pilot crew gets to continue on.