Run-from-RAM configuration with local USB backup

For the first part of your question, yes. You can use an md(4) device to run from. If I remember correctly that's what the old installation images did. Not sure how to do it but it should still be possible. You basically boot from USB, this loads an image onto an md(4) device and continues to boot from there.

I don't think there's anything ready-made to save settings back, so you will have to script something yourself. You could create symlinks for certain configuration files and store them in a separate directory. This directory is then mounted from the USB.
 
PfSense does something like this if I remember correctly. It has /var on a ram disk but all the important stuff gets synced to the permanent disk periodically.
 
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