The first claim seems to be incorrect:
There is a button to select all bookmarks. Also many other combinations of bookmarks can be selected by just pressing a single button.

If these buttons should not work, please report a bug.

> merging the contents of folders with the same names

Merging/Removing duplicate bookmark *folders* is IMHO a rather different task than removing bookmarks.
(Actually, I consider already the option to "move" instead of removing the bookmarks a questionable feature.) It would require a completely different UI.
Currently, it is not implemented, and I have neither plans nor time to implement it (if somebody really wants to implement it, please contact me by pm or on github.)

Concerning the bug that the moving does copying instead of moving: I cannot reproduce it.
Technically, it uses this function https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/bookmarks/move and if this does not do the claimed thing, I would say it is a firefox bug (which should be reported to firefox), or perhaps you have some strange folder which is e.g. immediately re-filled by some other extension or something similar I cannot reproduce.
But it seems that this part was removed from your comment, so perhaps you could not reproduce either.
If you can, please report details about the bug on github (or send me a pm).

Thank you very much!

Both (and much more) is possible in Expert mode.

Indeed, a folder is not empty if it contains a folder (no matter whether the inner folder is empty).
Theoretically, one could list those "recursively empty" folders but in practice there are several problems with that: One would have to couple implicitly selections of folders to be deleted (which might be confusing), the order in which folders are deleted becomes less transparent, and last but not least, some people would like to keep the "outer" structure of folders.
Instead of presenting a more complicated/confusing user interface, it was therefore a design decision to just possibly require several "clean empty folder" steps: The maximal number of repetitions necessary is the length of the deepest path consisting only of empty folders. In practice this should be far less than 10.