Alright here's my take on the DHMIS timeline. It contains graphic content so proceed with caution.
So David is Lesley and Roy's son, I'm sure we all know that already right? I'd like to think Yellow was a doll he used to carry around, one made to loosely resemble him. Yellow wasn't always sentient though, at this point in the timeline he was just a lifeless doll with a few prerecorded lines (the batteries imply that was how he functioned originally)
David follows a bird while it's flying. He ends up running into the street, and gets hit, but I'm sure everyone already knows that. Here's where I start branching out from the popular theories. I'd like to think David possessed yellow, but 100% by choice. Yellow wasn't something made to cope with the loss of David, yellow **is** David!
Roy doesn't know that though, and wanted to direct his anger and grief somewhere. Where better than this thing claiming to be his dead son? He is unknowingly tormenting his undead son. I find that idea so fucked up. Then by season two he actually managed to snap out of whatever weird mental state he was in, at least to a degree. He was starting to believe that really was David.
Now for David. Due to Roy's arrogance David is not able to process death correctly, especially considering whenever he and his friends died they were able to come right back. I'd like to think he did a bit of digging for information, and found where he was buried. He most likely unburied his corpse similar to how he did with Duck in the death episode, except his corpse was motionless, and decaying. Most likely infested with maggots. He didn't understand, and said he'd wait for the other to wake up. He'd be waiting forever though, it seems.
All ill say if the us supreme court decides to turn over the indian child welfare act and i don't see white people talking about it, especially white lgbt people, i will be pissed
You wanna ship yourself with a canon character? Do it!
You wanna make a brightly colored OC? Do it!
You wanna do a roleplay where your self insert is the one in power in an attempt to grasp onto the last bit of authority you have? Go ahead! You deserve it :)