I think you are overemphasizing the cases where it is better than TR, and kinda assuming that you have already built a deck that draws itself and has actions to spare. TR is good because it is two cards in one, a village + whatever you draw it with, which is balanced by the fact that if fizzles often. MM needs a lot of support compared to TR, which is balanced by the fact that it does something even when it fizzles, and that it has more potential.
Let's compare some cases:
TR + smithy = draw six cards dead. MM + smithy = draw three cards dead, copy your best treasure (a copper or silver at the beginning of the game, which is arguably the most important phase). TR clearly wins, if anything because of the extra cycling.
TR + TR + smithy = draw six cards, double any action cards from 8. MM + MM + smithy = draw three cards, triple your best treasure. It's not even close when you are building an engine.
The reason why BoM can be good (with the right Kingdom) is because it's always doing what you want it to be doing, independently of your hand or your deck. It can trash when you want to trash, it can give you actions when you've got card draw, it can give you buys when you've drawn your deck, it can attack, sometimes it can draw, etc. Both TR and MM can't do that, unless you already have those cards in hand/in play. If you want to trash, you need to buy a trasher first, and then you probably don't want to trash twice in the same turn; same goes for extra buys, discard attacks, etc.
And if you want to draw your deck, your first 5 cards must allow you to do so; if you are missing a key card, BoM will give it to you, while MM and TR won't.
Basically, the strongest cards are terminal, so you are going to need more support to bring MM to its full potential than you would need with TR. Which is fine, because MM can do some tricks that TR can't.
About MM plus KC, well, any game with KC is going to be kinda crazy, so I don't think that's a problem. A clause about "cards costing 6$ or less" could be added for balance, I guess.
The problem with MM at 5$ is the same as 5$ villages: they are competing with whatever you are using as your payload. In the end, only playtesting would tell whether this is in the "4$ cards that become really strong with the right support" (see bridge, conspirator, etc) or "solid 5$ card". And unfortunately I can't playtest it because I had to move away from where my playgroup used to meet