Sleep tracker

In the app on your phone you should be able to (health state, sleep and tap the 3 little dots). That said Garmin needs to improve its sleep tracking accuracy in my opinion (and others).

You can't easily vary the sleep start time but you can set the sleep end time to be later than you normally sleep and then when you wake up tell the watch "Not Sleeping" and it turns tracking off.

Yes, but what about if you want to do a nap during the afternoon? No way to check your short nap...

Correct, no nap tracking. You can turn on DND so the watch won't wake you but cannot track the nap.