At the moment this isn't possible but you could put those messengers in a webpanel so you don't have to leave your active tab.
To add a webpanel press "F4" --> click on the "+" on the left side --> enter url of your webbased messenger --> press ok
In Settings --> Keyboard --> section "View" you can ad shortcuts to fast access the correct panel if you like.
In case your messenger is shown incorrectly go to the webpanel and rightclick on the bar where the reload and homebutton of said panel are and select "use dektop version" or "use mobile version" as you like.
Of course this is just a workaround to your idea - so please feel free to post your feature suggestion in this thread so others can upvote your idea as you can do with the suggestion of other users to show importance for needs and wishes of the users to the vivaldi developers.
It buries new suggestions under a mountain of responses to earlier posts.
It does not. New suggestions are added at the end of the thread so all anyone needs to do is open the thread at the last read post to see the new additions since they last looked.
Personally, I would pro-actively moderate it to edit or remove posts that do not follow the instructions in the first post, and move implemented requests to the implemented feature requests thread. That does not seem to happen often.
Discussion does already happen within the FQ thread, probably too much. If you mean one thread for each FQ, then that would be more difficult to follow than a single megathread.
Users already know by posting that there the developers will see it. That's why gaelle started the thread.
You can wait for Vivaldi to implement this, or you take a look at this forum board: https://forum.vivaldi.net/category/52/modifications
All of your issues can easily be addressed immediately (and for most of them there is already existing code), if you are willing to put a little work into it.