It has worked well for months and suddenly I lost the XML editor. Where do I find it or, how do I get it back? (The program itself works all right.)
I don't remember what move I have done last.

The short answer:

1 - It might be docked
2 - It might be minimized in the docking area
3 - If you're working on a dual or multiple monitor setup, it could be somewhere outside your screen area (and there's a way to get it back)

Since the explanation for all of those it quite wordy (at least for me they are), I'd ask you if you have any insight about which one it might be, and then I can give you details about it.

Having lost both my XML editor and my Stroke/Fill, I un-installed Inkscape then re-installed it. Everything seemed all right now but, another little annoyane surfaced: the opening page keeps displaying a vertical rectangle that is empty, looks like a placeholder. How do I get rid of that?

I must reply to my own question. I happened to go to Document Properties and wondered where is that Page Border. To see what it does to my page, I clicked on it and, presto, that rectangle vanished. So it didn''t act on the page but on that pesky rectangle. Click and learn...

The rectangle is the page area. Just in case And the dialogs have probably been minimized, making the reinstall unnecessary. Just click on the icons on the far right of the window, before the snap bar, next time you have accidentally clicked on the small arrow in a dialog's top right corner, just below the dialog's close button.