Re 1: I would also rather have an easily replaceable battery, however for your use case I would think that a powerbank is more sensible than a second battery: no swapping/downtime and you can recharge both at the same time.
Sounds like fun taking photos with a powerbank connected to your phone ;-) In the past, I didn't have a problem with the one-minute downtime. Instead, I enjoyed having a fully charged phone within seconds.
Phone case with removable battery. I had a cobattery (now discontinued) case with my iPhone 6 - added a ton of bulk, but was useful to just swap batteries instead of charging.
