Adding to the thread is fine as I'll use what's posted in them to keep a track of things & will split out as needed if it makes sense.
So having a quick read through the initial points, the first is already on the list & is to follow on once the local library plug-in has been replaced (it has a few areas that leave it lacking compared to the existing ml_local.dll from 5.666 & so that needs to be done before adding additional view modes).
With the right-click menu, I've a feeling most of the issues you've having are due to running it under WINE as it is also meant to be skinned by default which does work on Windows. Trying to circumvent click areas & the likes I really don't want to try to do (which would involve making my own menu control & that's more effort than would be returned). If the menu was being skinned like I see it under Windows then it'd be easier to just bump the font size used for the skinned aspects & that might alleviate some of the issues. Ultimately its down to the OS to nicely handle such things imho than for me to attempt to hack around things & is why Quod Libet is going to respond in a similar manner as menus are meant as one off actions.
However, with the main thing that you're wanting to do (send the same file to multiple playlists), the better option would be to adapt the non-menu solution (preferences -> media library -> send to options -> uncheck 'Show all playlists on the send to menu' & you then get one 'library playlists' option & a dialog with the playlist to send to. It would be much easier for me to modify that (as well as you then just have one item to click) to have checkboxes next to the playlists (I'd have to make it a list instead of a drop down to make it easier to work with them which might be better to do anyway). That seems like it would be the easiest way to achieve most of what's wanted & not have to attempt hacks.
Having the send-to selection highlighted in the playlist(s) is a potential performance nightmare as it's ok with a single selection &a few small playlists but it'd get mess fast with multiple selections as send-to allows & with lots of large playlists (note: I've never tried Quod Libet so I don't know how it fares). Maybe the dialog option above can remove some of that pain but I'm not sure what benefit it would offer other than to see if it'll cause duplicates - if that's the case then it might be better for the playlist handling to do that when trying to send something (controlled via an option) as you then just take the hit when the playlist is changed instead of doing it in all cases as would typically be happening if the action isn't completed.
I think that's covered the main points you've raised & now I'll get some sleep, heh.
-dro