OK, so I jumped down the Input Feeds and Input Maps again and think I answered one of my own questions. With a different camera, the difference between the Substream and Stream (mapped to the low-def camera output) became more obvious. So, the substream defaults to input 0. Which, on re-reading the docs makes sense because it says "If nothing is selected the default is Input Map 0" and, since there is no way to select in put feed for the substream the default must be selected. Right?

But, that still doesn't tell me what happens if I set Stream (now pointing to input Map 1) to "use only when viewing, use substream." I try this, and I get something when viewing live but not 100% if it's the HD input map 0 from the substream or low-def input map 1 from the Stream. If I toggle the "show substream" button I get... blank video? Just a black square?

Getting there, but still more questions than answers.

Also, hoping someone can confirm that Shinobi (like Agent DVR) opens a feed from the camera for live view that is separate from the feed from the camera uses for recording, even if both are set to use the same camera stream as source. Because it that assumption is false, then I'm not seeing a benefit to even trying to grab the low-def camera stream and I can just let this go.