1- The popover has no a11y label, so it's unclear what the page titles being read by VoiceOver denote

Correct. The tools menu speaks its name when opened, we should have the same here.

2- Highlighting the back button only reads "Back button", but does not give any indication that a long press can be used to display the tab history

Correct. We should either
a- have 2 custom actions, the primary being go back, the secondary being show tab history
b- Have an acccessibility hint that says something like: "Double tap to go back, Tap and hold to show tab history". (I find this one a hijack of the hint.)

3- Long-tapping the back button will trigger a sound, but there is no verbal indication that the menu was displayed

Right, maybe fixing 1- will also fix this? The "good" thing today is that the focus goes to the background area which closes the popup when double tap, similar to the tools menu behavior (although the tools menu has a proper close button).
One improvement: today it says "Close menu, button". We could modify so that it says something like: "Hide tab history, button".

Thanks Kurt for the report!