Now, steelman argument is that DMARC is used together with SMTP, so it should be somewhat compatible to work in practice. But in this case there isn't fundamental conflict breaking SMTP.
I don't agree about that ZFS issue. Using whole disk isn't inheritantly wrong. When you have data pool separated from boot disks, using whole disks is better. No need to create partition table, when replacing disk. No worring over block alignment.
MID-s are used by MUA-s for referring earlier messages, tracking answers and so on. So any software expecting dialog (messages coming back) needs to deal with MID-s correctly. Missing MID-s show that said communication is one direction, because broken dialog has not been problem.
My take, as a postmaster for hosting company, who don't have any sympathy to gmail (that should be visible from my comments history):
Message-ID is absolutely MUST in production e-mails. You can send your test stuff without it, but real messages always have it. Not having Message-ID's causes lot of fun things. All somewhat competent software is capable to add Message-ID's, so lack of it is good indication of poorly made custom (usually spamming) solution.
Rspamd and spamassassin have missing MID check in their default rules, I am sure that most antispam software is same.
Testing with different ISP-s involved ISP provided resolvers and directly using archive.today nameservers. I know that archive.today has previously blocked common public resolvers, like 1.1.1.1. That case they replied with 127.0.0.4 or other loopback IP addresses.
> And not suffering, not being in pain was maybe what allowed her to begin to heal.
I think that is somewhat well known? I don't have good references about humans, but veterinarian James Alfred Wight (wrote books as James Herriot) has written about it, how lost cause lamb "miraculously" recovered after big dose pain killer (sorry, don't remember details, only his thoughs, how getting rest from pain helps).
Now, steelman argument is that DMARC is used together with SMTP, so it should be somewhat compatible to work in practice. But in this case there isn't fundamental conflict breaking SMTP.