Yes I'm quite sure it's 100% SP that is the payment for Witnesses.

This recent post looks like it fits in nicely with yours :D

https://steemit.com/scotbot/@holger80/scotbot-rewards-can-be-given-out-partly-staked

I wonder who will adopt a partially staked approach...

And yet, I look at other blockchains and wonder if they have staked versions. Does Bitcoin, Litecoin, Ethereum, etc., do all that at all?

Sorry I don't know the answer to this, lukestokes should be able to help.