You can access messages and the feed with it. I can't see any replies in the feed but just press whatever choice of modifier keys, mine is alt shift, then right and left arrows to navigate between messages and the feed, up and down arrows to see content. If you're in messages, modifiers plus n for new message and modifiers plus r to reply to a message. Hope this helps.
Yes, indeed. The quick actions won't change though unless you reset them back to the default state, even if new ones get introduced or the existing ones change their default values.
As I said, this is on a new computer so they're already at the defaults, so when I tried to add the keystroke I wanted I got the message saying it'd been assigned already to post to a feed.
Oh well, f gets me here to the forums but not as nicely as did f4.