Thank you for you feedback.

About 1, 2, 3 I try to do some thing close to Sage++ (a RSS extension was available on previous Firefox versions <57) and it is one single tree view in a side panel, and opening each feed in one tab.
But yes, I will think if I can make options to let the user choose how he want to display his feeds. But not right now, before I have to fix many defects, and continue to add missing feature was available on Sage++.

About 4, yes you are right. In fact I have done that (many check threads) but there was some issues, then I have deactivate this feature for now. I will reactive it soon as it will fixed.

It is tagged "experimental", because for now there are many things to improved ;-)