Yea can, but i can judge the quality XD

yes, you can plug your headset to it, I'm using my Arctis Wireless Pro headset via wired to it, since the all the next gen console doesn't have any optical out

Sound quality wise, once I've enabled the Dolby Atmos for headphone, it's sound as good as the original setup via optical
but it will drain the battery of the controller a lot more faster

For Iphone: Yes and no
Reason: The series X|S controller can connect to iphone but unable to detect by the Xbox App until iOS 14.5 release (the beta already has if you want)
Solution: Connect old gen controller via bluetooth to iphone and play

Issue i connect Playstation Dualshock 4 to iphone and play xbox remote is that, xbox will think the analog of DS4 = both D-pad and analog
Example: I play Forza Horizon 4 on xbox remote and i use the Analog to control the car, but it also trigger the in game D-pad which is also trigger the radio station =.=
For Android: Yes , no issue

Havent bought any yet XD

I have one hook for Dualshock 4 though, haha

Previously i will play remote if i dun have the TV privilege (when my wife occupied  )

Now i have Series X and S, i can just switch to either console/position/location to play

anyone of your controller's A button is a bit erratic at times?

mine sometimes wasn't registered, and sometimes the timing a bit off, especially when performing double jump in some games

haven't noticed the other buttons, since A is the one I used the most now as I'm playing Destiny 2 most of the time where you need to jump a lot