In an interview I heard, a party official made it clear than anyone who would like to run for president needs 300 delegates' worth of support. That's out of over 4000 delegates. So realistically, if someone else wants to be considered, they need to convince less than 10% of delegates that they're worth a shot, then tell the party that... like this week.

It all has to be settled before Aug 7 because Ohio state officials (who happen to be republican) set a deadline prior to the dnc convention (presumably they set this deadline after the convention was scheduled...)

This is all more or less spelled out in black and white despite the unprecedented scenario.