I finally did what i should have done some time ago: got rid of the mess created by an import and deleted all bookmarks. There really weren't any that were unrecoverable. Now things look tidy.
My recommendation: avoid the import function, especially from browsers messed up with too many, too cluttered bookmarks.
@ney-1 I think it would be more useful if focusing the search field would select the search string. Deleting or clearing the search string collapses the bookmark tree. It is much more useful to have it stay open with the current search results.
@askfor I don't know that there are established terms to differentiate between extensions from the Chrome web store and other kinds of modifications, like Vivaldi Hooks. Hooks is still technically an extension, as it extends the functionality of the browser, but it's a modification in that you must modify at least one file in Vivaldi's installation directory in order to use it: browser.html.
Such feature has been requested here. You can go there and vote for it. You can also take a look at other requests in that thread & vote for the ones that you like or even post your own ideas if they're not there already. 😉