I know about the guide regarding staying until biometrics, but as you said, they don't seem to enforce it (moreover, the new guides say that departing after filing should NOT affect the case (Just read the entire USCIS internal manual regarding re-entry permits, I'm that sad... lol)
I guess expecting someone to wait 10 months for biometrics in the US with the current backlog is not logical, and they know it. The whole idea of the re-entry permit is that you have to travel ASAP.
Regarding the zero flexibility thing - I don't have to try as I've read more than 5 appeals like the one I posted that were dismissed, all because the applicant wasn't in the US at the time of delivery to the dropbox, all during covid (the one I posted is from just a month ago). So you could clearly see there's no flexibility there. Or regarding the fees and refunds (covid economical hardships? not in their book). Even an appeal costs 675$ which is actually 15$ more than a new application. That's just messed up.
As for your last question - as I said, my country was in a "closed sky" policy during that time and I had to get special permission to fly from ministers themselves (yeah you heard that right). I had to do it before my visa expired (a week after I entered). Sadly enough, this only available flight was set to Friday, and by the time I landed it was already too late to mail the application. I mailed it first thing on Monday morning, but had to leave 2 days later, again, because of the repatriation flight. So just bad luck.
Obviously had I known I had to wait for it to be delivered, I would have used a next day express shipping. But no where in the instructions do they explain what "has to be in the US when FILING" means. I assumed, like so many others, that filing means sending the application.