I don't think so. Here are a bunch of things I think are not obvious:Quote fromWhat I like about combining creatures is that it's surprisingly intuitive rules-wise.
What relationship do their text boxes have to each other? Is one "first"? Does the combined creature have two text boxes?
What happens if they have conflicting CDAs? What if they have CEs in a dependency loop (and with the same timestamp)?
What happens if the creature transforms, or flips, or a morph ability of its face-down half is activated?
If half of a creature is face down, is it a face-down creature?
Can abilities be linked across text boxes?
I'm not saying we can't come up with reasonable answers to all of these, I just think you'd need to do a LOT more work than just having the word "combine" in the round rule :)
EDIT: Oh, I also disagree that Mary's "switch" is unambiguous. Does the new one enter and the old one leave, or is it more like transforming? Does it get a new timestamp like it would when it transforms? Are statuses and other properties not listed in the reminder text also preserved, like face-down, flipped, and phased out? If the thing you're trying to switch is no longer in your hand when the ability resolves, or the thing you're switching with no longer on the battlefield, what happens?