I've never had any issues using the M11 code and wouldn't expect that anyone would. But it is always a good idea to print out your reservation info just in case the hotel has "problems".

As far as the M12 code goes, I have to admit that I'm not a rate expert, but I don't remember it just being for Plats only. I would suggest looking at the rate details when you are making your reservation. As long as the rate details don't mention "Plats only", any Marriott elite member should be able to use it.