You falsely portray the situation. Since God is omnipotent, He can allow whatever He desires to come to pass, even to Himself. The question is not what CAN God do, it is what will God CHOOSE to do.
God comes to Earth in the form of a man. When He does this, He immediately makes Himself subject to whatever a human male suffers. For the first time, God has to eat, drink water, use the bathroom, suffer pain, etc. He doesn't have to, but being omnipotent, He can choose to. And He chooses to as a simple act of compassion.
Now that he is temporarily existing in the form of a human male, God is subject to the extreme agony of crucifixion. This is possibly one of the most painful deaths a human can suffer. It's true that God does indeed go back to heaven and continue His reign, but here's the point: the sacrifice doesn't lie in the fact that He died. The sacrifice lies in the pain He suffered. Essentially, He took on the pain that we humans are required to suffer because of sin. We are required to suffer the pain of existence in Hell. But since crucifixion is such a painful death, and since it quite possibly was as painful as going to Hell, God was suffering the pain of Hell FOR us so that we wouldn't have to. And therein lies the sacrifice.
It's true that God went back to heaven to continue His rule. But He does not forget the sacrifice He made. He chooses to remember it, because He knows he did it out of His love for us. We continue to spit in His eyes and mock and curse Him...but He still remembers out of His love that He endured the pain of Hell for us. It is also worth mentioning that despite being omnipotent, he bears the scars of his death. God, or Jesus, still has scars in His hands where the nails penetrated...and for time and eternity, He will always wear those scars. They will eternally remind Him of the "agony and ecstasy" of creating humanity.
Honestly, no Christian has ever been able to explain this to me, and I've read a LOT of apologetics.
The answer being fairly simple (pardon my long-windednes), I find that hard to believe.
I've heard that one before and haven't bought it. The fact remains that he is still, for that entire time, an all-powerful divine being, even if he is in human form. The amount of suffering that God could easily endure surely must be much less than, say, a person voluntarily subjecting himself to getting a tattoo.
Maybe this is implied:
* 'to my satisfaction.'