Ad 1. OK.
Ad 2. Hmm…
I admit that here I really understand the preservation of backward compatibility, however, I think I would change two elements as I did in my fork (
#1 (comment in lines 23-32),
#2 (lines 30-60)).
What I mean is the case when I want to apply changes to all xhtml files, however, but don't want to select them (I'm always in a hurry), when I can, after all, stand on the "Text" folder and know that it should make me mean "All XHTML files".
Now it doesn't, and I get the disliked message: "No text files selected in Book Browser!".
I believe that whatever decision you make will be fine because just this detail in the code is a subjective matter. The most important thing about this plugin is the very idea, execution, and stable operation, which makes – at least for me – TagMechanic a gem in Sigil.