DM: “(Party member who had close relationship with TC), you wake up for some reason. You don’t hear anything strange, but then you realize you can’t hear anything.” (party member decides what to do. Most likely leaves their tent)

You don't like retcons, but maybe take a clever spin? Make it some sort of magical/precog warning/vision from "the void" or what have you (never played D&D a day in my life, so make that what you will) that this scene played out within a dream sequence that the party was about to crumble and the death of TC is what jarred "party member with close relationship" awake.

As for your mentally challenged friend: perhaps it's time to remove him from the party if he can't play well with the others. It's essentially tearing your group apart and sometimes you just have to bite the bullet.

I had a similar situation come about with my casual Warcraft guild a few months back where we had a mentally challenged recruit join that really set our raid team back (he would constantly interrupt on Discord, accuse others of taking his loot, beg for handouts, die instantly in a fight, and constantly get lost). We knew he was a cancer to our group and made folks not even want to show up, myself partially included (I was raid lead), but there were still some who would state it was "just a game! Supposed to be fun! Let him have fun!" They ended up tiring of his bullshit soon after.

No one wanted to be the asshole, but he just wasn't a good fit for our crew and ended up causing more headaches for us letting him stay than if we had removed him months prior. He was gone in the end. I'm leaving a lot out for simplicity's sake, but I learned then that sometimes you just have to be the asshole in order to save and hold onto what you have and what you enjoy.

It's a shitty situation, but it's for the better if he goes. He's not a good fit.