That is an interesting use case. If I have interpreted your post correctly, you want to boot from a flash drive into a generic default OS or a persistent and encrypted / password protected OS. Doing that on one operating system is quite difficult as far as I know. However, you could dual boot two Fedora installs on one drive (I chose it because it’s what I use and I remember that you can set up encryption in the install. You could use whatever.). Basically, flash the installer to the drive you want to use, and to a second drive. Boot into the second drive and flash fedora onto the free space in the first drive, and enable encryption when prompted. The installer is a live boot (at least on the KDE spin) and will functions as the amnesic one. The other will be password protected and remember changes.
Debian and Fedora. I use Debian on servers and Fedora on my desktop and laptop.