In one of the other threads, there was a discussion about ZDDP being removed from oils. It could be that TC-W3 contains this. Most of that discussion focused on the zinc and phosphorous aspect. ZDDP is actually zinc dithiodiphosphate. The "thio" means sulfur. This is conjecture, but it could be a potential reason for it not being ULSD compliant. Since we're all (us U.S. guys) are pre-2007, we should be good to go.
I'm not sure why 2-stroke oil would have issues mixing with diesel fuel. Chemically, the two are similar. The hydrocarbons are just bigger. It might be the additives in the 2-stroke oil that separates out, though again I'd be surprised that there's that much difference. The base oil in the 2-stroke should mix in without any problem. There also could be a volume issue here too...people could be doing something like mixing in a can of the 2-stroke in a small amount of diesel fuel find that it's on the sketchy side of things. In a larger volume, like our ~20 gallon fuel tanks, it's probably less of an issue.