I don't think keeping the William/MiB theory alive is grasping at straws at all. I mean, as long as it's a possibility, it remains a possibility, and that's what makes discussing it interesting. I think at this point it is -more- likely that they are not the same person and everything is happening in the present. I'm not super invested in it being true, but until the show very clearly rules it out, it's more of a question of... why not?
But that still leaves questions about the logos and about the biggest "mystery" of all in the writing - if the MiB is just totally stand alone character who is an important person in the world and a VIP guess in the park, why doesn't he have a name for the audience? Why are we calling him MiB. Why are the episode blurbs calling him MiB. Why not just introduce who he is? That deliberate aspect suggests that his name and identity will have some impact, and that it will mean something specifically to the -audience- when it is revealed, because it seems clear that the people in the actual story know who he is.
Because of this central element, speculation continues. I think the speculation is intended by design, and so playing by the rules of what we have been shown, putting things together and seeing how they fit is part of the fun.