Yes, you can just use one at a time and it'll route audio input/output accordingly. They charge pretty fast in the case, too, so you could go a long time switching between each one at a time.

For long calls I often alternate them, charging one while I talk on the other one.

If you’re playing music or podcasts, removing one from the ear will pause the playback by default, and you have to tap play again. But with phone calls, the call just continues if you remove one for charging.

This is pretty much what I do when I know I’ll be talking for long periods.

Yes, often use that for 2-person content sharing too.

Yes.

Yes