Of course it might be that OP evaluated that and decided against it for good reasons.
Ethernet just works it's an established protocol and there aren't that many easy ways to transfer files other than it today that work out of the box and with any combination of hardware and software.
Serial I/O is common Hello World example for Raspberry Pi. I think you have to worry about less things than adhoc wifi networks.
> likely much slower
It's slower, but not much slower. The baud rate can be set up to 4,000,000, where a megabyte would transfer in 2 seconds. These "gifs" don't seem to be more than 10 seconds long, so they should be less than 1 MB, judging from random Giphy mp4s.
No, connect them via the USB and configure one of them to appear as a USB Network Interface. Point-to-point connection established. Plenty of HOWTO's available.