So just to confirm— it is okay to edit a little after the due date, as long as it hits word count/etc?

Mmmm. Yes and no. Edit as in fixing commas and rewording here and there: fine. Edit as in add whole entire scenes or do a total redraw because your beta hated it: Less than ideal.

We're on an honour system here, but my perspective is that if your work is an uncoloured sketch or a story that's not beta read and is missing an ending, and you totally think you can finish it in the next six days, but then (forfend against it) get the flu/end up working double shifts all week/have a sick aunt to look after, and don't finish it after all, your recipient gets an unfinished thing on the day of the reveal. Thus follows emergency pinch hits and bad feelings all around.

If, in the next five hours, you can't have something finished to the point where you'd be happy to get it as a gift, I'd honestly be happier to have a default now then have a scramble on the day the archive is supposed to open.