You are asking questions about more than one Calibre area.
Merge is Calibre.
The rest appear to be Plugin related. Calibre does not PUSH to plugins. THEY need to detect changes and PULL updates.
My experience with Merge, is it does not replace (non-blank) target metadata.
(I have been typing Ctrl-M for so long, I did not realize some of those other options existed
. I did discover I cold not ADD 'Merge" to the context menu )