@Jean-Sébastien3, from my experience:
1. I've hardly had any guests in the last few months because of COVID-19, but the few I've had have been fine. Unlike a lot of other hosts, I've not noticed any dip in standards from the guests who have stayed. However, I have been getting a lot of enquiries from guests who sound like trouble. I end up trying to dissuade most of them from booking, which kind of defeats the purpose of advertising on Airbnb in the first place. So yes, the standard does seem to be going downhill.
2. I have noticed that support has gotten a bit faster lately (compared to the first few months of the pandemic) but they don't seem to deal with stuff very effectively and you have to explain over and over again.
3. I've not opened any resolutions but you only have to browse the CC to see many other hosts having similar problems to yours.
4. Likewise, not had to deal with this lately.
5. This is the way it's worked with Instant Booking for a long time, which is the only reason I use it. I didn't ever want to use it and didn't at the beginning but then Airbnb really started to push it and I found that I dropped off the search results if I didn't use it. Then there are also the star ratings etc. (weirdly though, lately I have seen star ratings appear on some non IB enquiries, but not sure if that is just a technical glitch) and the penalty free cancellations.
This is nothing new, as they are incentives Airbnb uses to make more hosts use IB, but you are right, it's not fair. ALL hosts should be able to see a guest's star ratings. After all, any guest can see ours. All hosts should be able to cancel penalty free occasionally, if they feel uncomfortable with a booking. Unfortunately, I really don't see Airbnb changing this.