Q: I understand that with one Krarks Thumb the replacement clause makes it only trigger once, but if you have two or more (with Mirror Gallery) wouldn't the extra flips trigger the others? So, would this cause the infinite loop, or would the replacement clause protect it from this, too?

A: Replacement effects do not trigger, they just replace the specified event with something else. And a single replacement effect can only be applied to a single event once. So this does not create a loop. What happens with two Thumbs in play is that when you would flip one coin, you flip four and choose one.