Pi dual LTE 4G setup for either redundant or bonded cellular using Speedify

I am looking at a project to use a Pi4 with dual 4G and like the hats and 4G modem. It says 2 can be used. Questions if someone has done this or understands what issues I may have with this project.
1: Can the USB2.0 ports be used for power with the included usb right angle cable.
2: Can I stack the 2 4G hats without issue.
3: I was thinking of using an external router for extra wifi clients or use with POe devices.
4: I was looking to use Speedify which I see works on Pi and the boned 4G can be shared.
5: I would look to maybe add a video device, either a HEVC encoder or a USB3.0 capture card.
6. The box would be battery powered for field use, so regulated 5V will be used, but wanting to know if this could within pi be set to auto boot on a new battery connected and it there would be no issues with just swapping a battery. I imagine to program it and then it works on startup to encode and via wifi, I could access settings of the encoder over its own wifi network.
It is an interesting project but topical for remote filming.
I am interested to hear some feedback if this is a good option.