Hi,
On the topic of how to use the USB category in the App Center to distribute flatpaks and OS updates offline, there is some documentation here: https://community.endlessos.com/t/release-endless-os-version-3-4-7/7544
However since you are using a live ISO you would only be able to use offline app updates not offline OS updates, as Endless doesn't support OS updates of live systems.
There is also a blog post about using the same feature from the command line: https://blogs.gnome.org/mclasen/2018/08/26/about-flatpak-installations/
