All in the execution. You're trying to make it seem like this impossible-to-grasp concept and bad writing because you don't like the idea.
The show has clearly been seeding plot points about the past, from the critical failure to the MiB's history to literal flashbacks in this episode and the fallout between Ford and Arnold to Dolores being the original/oldest in the park
All you have to do is match-cut between William and the MIB and people will understand they're related and the same character. The show literally does in this episode (let's go back to beginning, cut to William)
Or that's just a transition? And doesn't mean mib=william. The end of the episode is way heavier on the scale that mib isn't william than that cut to william. If you can't see that then I don't know what to tell you. It doesn't rule it out, but comes pretty damn close.
And everything you are saying is true, but I think they are just developing the backstory and the critical failure and mib origins will obviously come to light, but that doesn't mean mib has to be william. I know you are open to that not being the case, and I'm open to it being the case, and yeah if I like it or not depends on how they execute it.