I am 100% sure that Sota is dead.
And the current chapter is just another dream.
And the "him" that Sota (from the dream) warned Tsubasa about is definitely this boy Kenta (or somebody who is using Kenta's appearance).
This is my hypothesis about the car incident:
The family went to the alzhemeir hospital to get grandpa checked.
On the way home, mom mixed in drugs into the drink and have the whole family drinks it to have a mass suicide.
Somehow, Sota realised something is not right and managed to find them unconscious inside the car/van.
Panic & shocked, Sota start to drive the van to rush them to hospital, but ended up in an accident, Sota died there.
About the real Kenta.... I feels like Tsubasa's mom may actually killed him...?
How did Tsubasa even get over to awesome Sota's? Sota's place looks pretty far too. I'm sure Tsubasa's fam never took him there and I didn't see any mode of transportation. I feel like it's going to take another woozy turn.
Also, that pretend kid is gonna grow up entitled and abusive, Sota should fix that, he'll turn into a punchable person.