Well Xemnas is an interesting case all on his own.
Yes, from a loophole Xemnas could have found out about the LOD's trick from ME or MX just for the fact that they are Masters.
could have Xemnas have that ME and Aqua memory and deduction about Ven...Yeah, there's a chance of that but that just seems way to conveinet to me.
Not to mention, just as we recently learned Roxas was a potential vessal for Xehanort and yet Xemnas didn't care wether or not Roxas survived the final fight with Xion because one was going to absorb the other, i believe he also gave her boost in power as well...Kind of sounds like self-sabotage
edit, I don't think Xion would have been a good vessal for Xehanort, even if she did win against Roxas. and i don't think she was mentioned in that regard either.
Yeah, Xemnas loves him some violence!