New to HTC and S-ON/S-OFF: Can S-ON users restore a TWRP backup?

I admit it; I am totally confused by what S-ON and S-OFF allows/doesn't allow. I did read this, which was helpful, but it still not clear to me.

I know with S-ON that I can: (A) Unlock the bootloader; (B) add TWRP as the recovery; and (C) Flash a custom ROM. I know this because I have done all of these things.

But I don't know how to (if it is possible to) restore a TWRP backup. I tried and failed: when I hit "Restore" TWRP replied

Code:

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Cannot restore System -- mounted read only

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Am I supposed to mount the System read-write to restore my TWRP backup? Or is it just not possible?

I haven't bricked my phone (yet) -- though it won't boot into a TWRP-restored ROM either. I can get to download mode, recover, and the bootloader.