all those years without blood would make him ravenous for it and bring his animal instincts out.
Good point. As for his being asleep, I say he was awake until he was in there long enough to start to go into a weird mental state a bit similar to what people in sensory deprivation tanks experience. I know they get very real-seeming hallucinations. A state between waking and sleeping is probably involved... I'm not sure a vampire is physically capable of sleep.
Barnabas agreeing to be destroyed isn't implausible, it's just how we get from there to 1967 that throws me. Perhaps if they'd tried to do something to convey the unexpected horror of being trapped in the coffin, and not just a brief moment but an extended look at it.... that might have bridged things for me.
This is the way the world ends
Not with a bang but a whimper.
This is the kind of answer that might get me to appreciate this moment of the DS story. It's a start.