There were a couple of reports that people had to temporarily put a T-Mobile SIM in their unlocked phone if they were AT&T customers to get the beta. It can even be an old SIM with no service apparently as long as you are connected to WiFi.
Yes, but it's hit or miss on whether you get the offer to upgrade. I have an unlocked S10+ on Cricket and mine showed up late yesterday afternoon. My wife has the same phone purchased at the same time in the same transaction but hers has yet to receive the upgrade option. Be patient and hopefully it will show up for you. To be honest though, there's nothing in the new version that I couldn't have lived without.
And yes. My PNC, Fifth Third, and Chase Mobile banking apps are all working correctly without issue.