I had a mail relay that used OpenBSD; it's LAN address was .239. I have since made another relay, and used the old OpenBSD box to make a Debian 12 box for use in our shop, and it also uses .239. (I can't control that, corporate has a strict mapping system in place) I can ssh and sftp from a terminal just fine, _but_, I can't sftp from Midnight Commander, my go-to file util. This leads me to believe that perhaps MC uses a different key file than the system (?), and that perhaps it's using a key meant for the old BSD box. If that's the case, where might MC keep its keyfile, as I just can't find it. Any insight would be appreciated.

Checking man mc says the standard ~/.ssh/known_hosts is used, and also states that an incorrect key will result in an appropriate message being shown...

I'm not sure what the problem is at this point, D/12 doesn't keep logs how I'm used to, there's no auth.log, and none of the logs indicated what the problem was, so I just wiped the disk and installed D/11, and it all works now. Must be some security thing, and how MC announces itself, I don't know how that stuff works.

The change in logging is noted in the Debian 12 Release Notes.

Specifically, under "Issues to be aware of for bookworm" is Changes to system logging:
Sticking with Debian 11 for a few years allows you to defer the decision, but ultimately your choice will be re-installing and configuring rsyslog, or switching to a systemd-free derivative, such as Devuan.

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All of the logging stuff aside, I still don't know why debian 12 refused MC's connection attempts using sftp on port 22

debian itself does not refuse anything. Probably the default configuration is different or stricter. You can use journalctl to read the logs, or you can use rsyslog if you wish, but that is not really suggested.