Regarding your #2 question, the procedure is simple. Leave your US sim set to roam, but select a non roaming carrier for it in Settings, Cellular, AT&T line, Network Selection, Automatic Off. You must then choose a carrier that you have no roaming service with. Probably one of those towards the bottom of the listed carriers. Once selected, your Status Bar at the top of your screen for that line should change to No Service, then after a few moments to Using Cellular Data. This function is called IMS, and works well with a second line providing data. Your AT&T line is using the data line as a pseudo WiFi carrier. Provided that AT&T gives free US WiFi calls, you will have no charges, and on your account those calls will record as made on WiFi. I'd review what the charges are for local (UK) WiFi calls are since your voice line will be active.

You can practice the setup prior to leaving the US if you have a second line available for data. I use the First Responder (313 100) network for my voice line non roaming carrier, since I cannot roam on it. Or, Verizon or Sprint should work in the US for you also.