@deletethis abject apologies. I did so many things wrong here that a concise explanation won't be possible.

I had chosen e.g Black Day and Dawn from the coop tab of the client Play page, in order to download the maps. In my earlier post in this thread, I referred to those as "regular"; likely yet another mistake.

As I try to re-trace my steps, it becomes apparent that a) I'm still not following the directions in your linked thread, and/or b) there are problems with my setup, resulting in my inability to save. I'll pursue that elsewhere.

In my earlier (6 weeks ago) struggle to resolve my issues, I found those old threads. Not finding a thread on the forum specifically saying it was fixed, I ASSumed it was still broken. (No, I didn't review subsequent patch release notes.)

So, a) I thought that what the OP was attempting to do was similar to what I was failing to do, and b) I thought it was still broken. Apologies for the additional hot air, but you did say "why".

@inspexor Possibly I'm mixing topics here so assume this source is unreliable.

I've had good luck saving skirmish games after launching with init_coop.lua

So far I've not been able to save and load a regular campaign mission. I can't remember the exact behavior now - it has been ~6 weeks since I tried. I'll try to add links to a couple of threads that looked pertinent to me.
link load-save-broken
link load-save-under-repair

But my attempt was with the regular campaign. You mention a custom campaign so your experience may be different. @deletethis should have a better idea than me.

@thecore: Many thanks! Both for the mod and the fake-fullscreen guide link. I absolutely will try out the mod. First I'm trying to digest as much as I can from that other thread. (Sorry, I move really slow - I fear that means I'm not gonna get far in faf.)

gog username: thtinker
email same as faf; I assume you can get that from profile if needed.
We'll see how the allow chat from everyone works out - I'll post here or pm (dunno how?) if I decide to revert that.

Your post seems clear and simple and I'm taking it literally. But I like to make up complications:

1. Yes, I have a gog account and I made the settings public.
2. Yes, I do own Supreme Commander "on" gog.
3. But I have NOT attempted to link my gog account or scfa copy to faf. At present I'm using the Steam version for that.

(I never got sound working under Linux on the gog version of supcom1. Didn't install SCFA.)
(At some point I would like to get the gog version working with faf and be fully DRM-free.)

Oh, duh (headslap). FINALLY I grasp your meaning, which should have been clear on first read.

OK at some point I'll report my results. I know I'm asking a lot of Wine / Proton, but it would be a huge step forward to get that second monitor working.

@Sprouto thank you. By actual game, you mean the Steam (or GOG) version of SCFA?

Apparently I mis-read the Secondary Adapter video setting as Secondary Monitor. If I read @Sprouto correctly, that setting doesn't work in faf?

I had a tentative plan to try that again at some point in the future and record my (Linux/Proton) error messages to post here. But it sounds as if that would be pointless.