As a follow on, I found a couple minutes to play a bit with the pretty-print and validate: the pretty-print does seem to work on a selection; the validate seemed to use the whole document, even when I just had the small segment selected. Though since I’m not good at XML, I could be wrong in my interpretation.
However, while the XML Tools pretty-print does work with individual selections, I had forgotten you said there may be thousands, which means it won’t be very efficient. I think the three-step search+replace/xml tools/search+replace is probably the best in-Notepad++ option.