I'm not sure which BB shell dimensions your '09 Flash Carbon has, but when it's Ø42x72mm, then you should order the THM BB30 MTB bottom bracket.
If Ø46x72mm, then the THM PF30 MTB version. If it's 68mm, then you might need to resort to a THM Road BB.
I don't see why a double ring setup wouldn't clear the chain stay and with XX1 you'll only use a single narrow-wide chainring. A 36T is large enough to clear the outside diameter of a 104BCD spider, so it doesn't bring the chainring further towards the chain stay in compared to a double setup. In fact, you might be able to mount it on the outside position, depending on how it influences the chain line.
Your THM cranks should end up looking something like this:
You'll be fine in upgrading to a THM M3 crankset, also with a 36T chainring XX1 setup. Just make sure you get the correct specification BB.
Here are a few images of chainring clearance with your desired setup on an Open 29er frame: