Yes
In practice likely never. If no port specifies any PVID, the PVID should default to the ID of the VLAN containing the port as untagged. Yes, just specifying the netdev should be sufficient.
Both should work.
As above, buggy implementations aside, <portname> == <portname>:u*.
The only use case for multiple bridges I can think of is either "retagging" (e.g. lan1 tagged with VID 10 bridged to lan2 tagged with VID 20), in this case you'd create a bridge over switch.10 and switch.20 or involving other netdevs (tap devices etc.) not being part of the bridge. In practice I found the single vlan aware bridge approach to be the most reliable/widely supported one.
Probably due to a lack of understanding. Personally I didn't yet encounter a practical use case for it
Correct, layer 2 (bridge) and 3 (IP config) are strictly separated
Yes.