from my basic knowledge,

Q1: Stacked should be better which allow you to use both ipv4 and ipv6 but old networking devices can cause trouble with ipv6 then ipv4 only connection will be better.

Q2: I can say our networking devices will do all the job automatically...

Q3: Most of the thing can do for end user is configure router and end device.

Q4: Only for UniFi user and with the same router. Different router or isp can have different configuration.

Q5: 2 possibles, either isp doesn't provide ipv6 or the router might not supported or not configured to allow ipv6