Unless, there is something more pressing, fixing this would be nice for CDS

I assume you mean the "back button" of Chrome, not the Android back button.
Chrome doesn't keep history of WebAPK is a known issue, and  We didn't fix it because it is complex. It seems impossible to know which WebAPKs to return to if multiple WebAPKs can handle the previous URL. I am not sure what the expected behavior is, but if we expect the intent picker to show for this case, it might be fixable.

This would be nice, but I think you can use recents to achieve approximately this anyways? (Since the WebAPK would still be in your task stack.)

I don't think that this is a problem anymore now that we show the minibar instead of bouncing to Chrome when a user navigates out of the WebAPK's scope ( Issue 680167 )