I wouldn't look for major upgrades in the Chess.com computers, especially not the Java computers. There are many much stronger engines available today, including some like Stockfish that are available for free.
You can do what you want playing against any engine. Just load your game and pick the spot where you want to play. You can rewind, take back, whatever, whenever you want. You can even dumb down the engine so that you have a chance.
I think that would be great feature. I usually lookup the point of no return in the archived game and would be great if I could rethink it and do some other turn and then continue to play from there vs. computer. I see that there is a "Finish vs. computer" feature, but once you load it it doesn't let you to rewind the steps back, does it?
Is it a problem for a comupter to come into play at an arbitrary point in the game?