Q: Open files in existing editor

Yes, I had, and it worked. For what it's worth, it worked in earlier versions just as well. It tries to open the file in the first instance. Sometimes, when some files open in the first instance are changed outside the editor, SE throws a dialog about that and asks whether to reload them, it fails to open in the first instance, and then it opens a new instance and loads the file there. That's arguably better than simply waiting for the dialog to be closed (by allowing or rejecting the request for reloading). I'd prefer if it waited for the response and tried to load the file after but that's purely taste and preferences.

My problem is that I frequently work on more than one project at a time and even if I'm focused on a single task, I keep the working environment for the other projects open on separate virtual desktops because it is much faster to have just another virtual desktop and keep the sets of external editors + file managers + SQL Editor instances running all the time for each project and simply switch to the proper virtual desktop than to constantly having close and reopen the project files in the external editor, change project paths in the file manager and open/close currently edited files in SE. The current method works very well, but only for a single set of cooperating applications (ee + fm + SE).

The chaos and confusion arise when the first instance of those apps is started eg. on virtual desktop 1, then I create the same set of open apps for project 2 on vd2 (and repeat for project 3 on vd3, etc.), I start working in the external editor on vd2 and I want to pass the file to SE. I'd expect it to open in SE on vd2. Instead, it opens in SE on vd1. If I'm lucky. Otherwise, the SE on vd1 first throws a dialog, and then it starts a new instance on vd1 to open the file in it. And that's when you have 2 virtual desktops. Imagine having 8.

Now, you might say that this is very far from the common practice and can be safely considered power use. I'd definitely agree.

Another, probably unrelated but amusingly similar issue is that when I make some changes in the Options and then apply them, all the currently open instances of SE will leave the safe haven of their dedicated virtual desktop and rally to the one the Options window is open on.

I think "power use" is a mild term for that. I'm pretty sure multitasking and working on 8 different projects simultaneously is a rare use case. Number of monitors and additional virtual desktops has some limits. Have you thought about using Docker Desktop or something similar to truly virtualize the systems? You could still have files shared between them by mounting projects root folder located on the host system as a network drive accessible from virtual desktops, or using a real network share for project files.

Luckily, the set-of-8 was not a recurring event indeed, alas, it wasn't unique either. 3 is common, with frequently climbing up to 4. But the trouble becomes obvious as soon as I start the second set.

As for the Docker Desktop, that would be an overkill for my goals. The built-in virtual desktops in Win10 were/are doing a remarkable job, well, admittedly only after a couple of months, when some not-so-dumb developer/designer had the divine spark and remembered that if those desktops could be renamed instead of having the generic names of Desktop1, Desktop2, etc., then they'd have the slightest hope of actually being useful.

Anyway, thanks for the hint. For now, I'll bridge this little gap with an AutoHotkey script. I'm confident it can be done in a dozen or so lines. I'm also pretty sure that ultimately I'll end up creating another monstrosity of code that will try to predict the weather, the lottery numbers, and do a couple of other useless things at the same time so that it does not get bored while waiting on some juicy file paths to process.

UPDATE: Well, the file is 14 lines, but two of those are block-closing braces and one of them is the EOF empty line, so I don't count those. It definitely smells like a hack it is. But it works. I'm satisfied :)