I’m kinda amazed it took this long, but valve finally added:
Added in-game keyboard shortcuts to open the Main Menu and Quick Access Menu.
Default Main Menu hotkey: Shift+Tab
Default Quick Access Menu hotkey: Ctrl+Shift+Tab
Customizable in Settings->In Game
Kind of. You could only open the menus IIRC when you were not in a game, can’t recall the exact key bind though. At least, this was the case when I tried replicating the gaming mode session on my PC (I haven’t actually used a keyboard on my deck either). It was quite strange.