I'm not sure what you are trying to achieve. I have a computer running Mint Mate (13) with Cinnamon desktop and it works just fine. It has extra 'baggage' over a pure Cinnamon install, of course it does, and that might slow it down fractionally, but it doesn't stop it from working. I have certainly heard of people who installed compiz on Mate getting conflicts with Cinnamon, but not otherwise.
So why can't you just leave things as they are? Why do you have to start over just because you have a few extra files lying around? I don't understand the problem I guess, maybe you are having problems with Cinnamon? If that is the case you don't say so.
As regards triple booting, if you install Mint Cinnamon it will take over the booting from Mint Mate (unless you alter it so that the original Mate stays on the mbr) so it won't matter. Why will that interfere with W7 booting? I don't get that either.
Of course if you are talking about a Uefi installation then in all probability, your house will burn down, your dog will get mange, your wife will contract the black death and your legs will fall off
, but with a normal computer all should be well.
Fujitsu Lifebook AH532 Laptop. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Ubuntu12.10 (Unity), Mint14 (Cinnamon), Manjaro (Xfce).