I just want to know whose fucking idea it was to schedule the DNC convention and debates AFTER the cutoff for the candidates to be officially on ballots in several states.

As far as I know there weren't state laws changed to make this happen; it's been this way for a long time. So literally the DNC (and GOP) have been sleepwalking their hand-picked candidates (pretend the Primary matters if it makes you sleep better) into elections because they didn't think it would be possible that their patsy could be too old, too ill, or too unqualified.