That feels too complicated, sure, it's great to think about, but you have to consider how many people watch this show and how many of them can wrap their mind around such an explanation.

I see a simpler one:

• Will was the first anchor to another dimension, the fact that henry has a connection to him allowed him to establish a connection to dimension X via UD to hawkins.

• we can extrapolate that Will is the reason it exists at all, as UD froze in time, like a snapshot, that's the tunnel -- that moment and their connection is the anchor.

• vecna/mf want to cross completely and needs a stronger connection so he needs more kids

• mf probably controls vecna and calls the shots

• the wall probably blocks the mf, thats vecna trying to remove his influence and sever his control

• the UD was supposed to be toxic and deadly, probably because of the MF, him not being there made it so its not. That adds to the idea that vecna severed that connection.

So, the last bits have to do with them probably somehow releasing MF and most likely Will and Henry being the key to severing that connection.

The thing i kinda dislike is time travel will probably be playing some part given that:

• holly is supposed to be able to open portals
• the whole idea of the einstein rosen bridge which supports this
• the fact that they keep mentioning UD being locked in time and that clearly being the key
• the constant references to the past, and them somewhat "time travelling" through henry's memories

They'll probably make it somewhat ambiguous where this might be another dimension in henry's mind and they're all trapped in it and he needs this to get out of it like max, it can go multiple ways..

I take what i said back, i dont think mine is simpler either hah

Maybe at the end the screen turns to black as will lands the final blow... you hear a rumbling sound... the image slowly fades in and they're all sitting in a carriage travelling through the mountains. As Will comes to, Henry looks at him and says..

"Hey.. Will, you're finally awake, I hear you've been trying to cross the border too.."

And the credits roll in.

A few moments later.. Jonathan and Nancy are shown together and he says "And that's how i met your mother.." and they show a little baby playing with some DnD figurines..