So there's been a 1-week temporary delay in implementation. I shall return to it next weekend, with all four of my source devices capable of delivering Dolby Vision plugged into four (of the seven) ports of the A1080 that support HDCP 2.2.
Actually, turns out that all seven ports of the A1080 support HDCP 2.3!
Anyway, the new AVR arrived today, along with the 6-ft USB cable allowing me to use that old phone charger in a wall power socket to power the splitter, avoiding the USB port on any of my devices which appear to only have power if the device itself is also powered on... which is undesirable.
So the surgery is now completed, and apparently was a complete success. The two Oppos (103 which is really obsolete and pretty much unused, and 203 which plays discs as well as through which my WMC and 720p/1080i HDTV goes) are plugged into AV1 and AV2 of the A1080. Roku, ATV4K and Shield are plugged into AV3, AV4 and AV5.
As prescribed, HDMI1 out of the A1080 goes to the splitter, which has HDMI1-out going to the TV and HDMI2-out (as 1080p) going to the A16.
The only wrinkle is that the Yamaha AVR does not actually pass-through HDMI audio out either HDMI1-out or HDMI2-out, except when you enable that option. Actually it's described as "enables/disables the audio output from a TV", as clearly they expected you'd not be feeding a second AVR with full HDMI audio when you've already got this one to do just that. So if you do turn this switch on what gets passed-through out as audio via both HDMI1-2 is downmixed 2.0 stereo to feed what are expected to be just two speakers in the TV. Obviously this is unacceptable for the intended use of the A16 now being fed from a split of HDMI1-out.
However, the solution is simply to configure the AVR to have "HDMI standby through" set to ON (actually AUTO, which is better). This is ideal, as it passes through FULL source audio and video received from whichever was the last input selected before powering down into standby mode. And as I mentioned in an earlier post, it's even better, in that the INPUT button on the remote is kept active during the AVR's standby state. So you can even change inputs... while the AVR is off. And in fact there's nothing that the AVR needs to do at all which would justify it's being powered on, when using the A16 for sound. Video goes to the TV and audio goes to the A16. That is exactly what I want... A16 powered on and AVR powered off.
And conversely, if I actually want to just "watch TV" and listen through speakers, the A16 is powered off and the AVR is powered on, supporting "2Ch Stereo" downmix to my 2.0 speakers from whatever source is selected (for TV it would be the Oppo 203, but it could also be for streaming sources).
In summary, it all looks good from here. Even using apps on the TV and feeding audio to the AVR via ARC/eARC is facilitated. If I want to listen to sound through the 2.0 speakers the existing cabling supports that, and audio goes back through the splitter to the AVR for handling. If I want to listen to multi-channel sound through headphones I do have to move that HDMI cable going to the TV from HDMI1-out of the AVR to HDMI-out of the A16, and change the audio source for a preset to "eARC". But it works.
There is a bit of an epilog here though, and I'll resume work tomorrow. There did appear to be a lip-sync problem when using the Shield and Hulu source. I don't know if that is going to be a problem on all sources, from all three of my streaming devices, and/or my two Oppos. The A1080 does have lip-sync adjustment capability but I don't know if it's global or local by source or what. Needs to be investigated. But not surprisingly it does appear that there's a bit of a time delay for the video getting through the spitter and on to the TV, vs. when sound is put out by the AVR (at least with the default lip-sync delay value of 0ms). I'm sure this can be dealt with, but it might have a different lip-sync delay when listening through the A16. We shall see what needs to be done.