I'm not sure this is necessarily a 'fix', but I was having a very similar issue as what you describe, but the converse. I have two monitors - when I run Outlook on the one that's in a landscape orientation, I can resize the folder pane with no problems. When I move the outlook window to the other monitor which is in the portrait orientation, I can only reduce the size of the folder pane - it won't resize larger beyond a fairly narrow size. I discovered through another forum a sort of a workaround... (This assumes the reading pane is on and is set to be on the right.)
When Outlook won't let you resize the folder pane the way you want, drag the left edge of the reading pane as far left as you can. For some reason when this is done, I can then resize the folder pane. Afterwards, just resize the reading pane back to where you want it. This is continuing to work after a few restarts of Outlook. May be worth a minute of your time to check if it works for you also.
Interesting, but that has no effect on the folder pane here. It's still a hard wall when you try to move it left (without the earlier dumb workaround of mine). Do you think it's maybe some byproduct of your dual monitor setup (just one here) that's aiding you somehow?
I'd really like to find some resolution to this, since it's also broken in the "new UI" in 16.0.14301.20004 out today. Further, my dumb workaround nullifies the new UI in not only Outlook but Word, Excel etc. That is, deleting those keys resets things to the old UI, and if you want the new UI again you have to re-enable it again and then not use my dumb Outlook workaround. But, of course, if you don't you're back to the Outlook problem....
Hmm... quickie test on a 24" monitor (2 monitors, Outlook is on #2) The width of the message list seems to be the key - with the list at its minimum width - i can change the folder list to cover half. The reading pane get shrunk. So... shrink the message list, adjust the folder list....
Diane, your third screenshot shows what we're after (though only if your message list doesn't need to be that wide), but are you implying that your preceding maneuvers are what allowed you to get that far left? Or were you already in the state where it wasn't a problem to get that far left? When in that state, I've tried all kinds of dragging of everything but can't find anything to "unlock" the ability to get that far left, which is why I resort to deleting the Registry items.
Maybe that would have worked, I don't know, but nothing previous did (for me, though we may each have different expectations about this so may be talking about two slightly different things) except the Registry.
And as of 16.0.14322.20000, not even that works, which is not a good sign at all. The "experiment" may be over.