Regarding your suggestion (3), I will try to look at this in the future (I write that in my todo list), but don't expect this soon. You know...
I know, and I completely understand. For now, my xml sitemap plugin uses a dedicated polylang get languages routine :)
Regarding the admin language filter, I found that I was clicking the radio buttons and not the text. When you do that, you'll notice that you can activate all radios at the same time but nothing happens... Only when you click the text, the page refreshes and comes back with the filtered posts. Not a big issue but still, it confused me a bit ;)