Well yeah it's all good, that's what a lot of this is, theory/speculation.
Just because he doesn't attack them doesn't mean he's benevolent. I can see what your saying, but it's not true lol. He doesn't have to fight them right away to still be their enemy. Have you ever heard of mind fu^king someone? He can do a lot of things that are of malicious intent that don't involve direct physical violence.
As for the whole Vanitas seed plan, it just doesn't seem logical(I'm not saying that you aren't, just that). I don't think that time is in favor of that notion. It means that before everything went down at the Keyblade Graveyard in BBS, MX would have had to given it to Van. Wouldn't that take away some of his strength, why would he do that right before he has to go fight Terra? I don't think that MX would have given Van any light at all, Van wouldn't be Van(pure darkness) then anymore, making his duel with Ven to forge the Xblade irrelevant. I don't think a little more darkness would have hurt Van in anyway either.