I made them howitzers because I needed to take some creative liberties. There was no way I'd give it the E100 gun, especially with the VK100. Howitzers are not as damaging.
You are right, the Maus's job is to take hits. That's why I made the guns howitzers, because a normal 15cm gun would make the E100 and VK72 irrelevant. Giving it a howitzer makes it much less effective, and really only a threat when dealing with soft targets. I don't know why you keep thinking people would run the howitzer on the Maus, it's only there for fun and most will stay with the 12.8cm.
And yes, I have not played the Maus, but the VK 100 is along the same lines, the only problem being the center mounted turret. And since the VK 100 is one of my favorite tanks to play because of that armor and gun, as well as watching lots of Maus replays, means that I've got a general jist of how they play. Once I get the Mäuschen (which could not be very long), it's essentially a Maus except for the large cupola on top.