I think the ipsw just enrolls your mac into the beta so it can download the beta from the settings "software update". This is a new way that Apple introduced to download the beta on macs (which is kind of how they've been doing it with ios/ipados), but it will install over your current installation.

Yes, that could be an option, though a kind of painful one. I would have to repartition my drive and reinstall everything. In any case, I got the USB-C drive to work. I was about to purchase a Thunderbolt drive to test with that, but turns out I don't have to. Details in the discussions.apple.com thread link I provided