Below is the TV Adapter screen on the Genie 2 software page. As you mentioned, it has 5 settings that the audi can set for you.
It sounds like based on your description, the ON app has its own default of 16 that may override the default set in the Genie 2 software. So what if you don’t use the ON app at all (turn it off in your phone) to avoid it overriding the Genie 2 default value as a workaround? I personally just don’t use the ON app at all, so I never run into this issue with my TV Adapter volume. I just cycle through my programs and volume via the HA buttons. I find that to be much faster than through the ON app.
There is no TV Program per se. As you can see in the second screen shot, the 4 programs available on Genie 2 are for the other stuff. If you don’t pair your OPN to a TV Adapter, you’ll only cycle through up to these 4 programs only. If you pair your OPN to a TV Adapter, then you’ll notice (hear) the TV Adapter program automatically added to your OPN as you cycle through P1, P2, P3, P4, TV Adapter. It’ll also show up in the ON app accordingly But there’s no TV Adapter program in Genie 2 to set because as you can see in the second screen shot, there’s only P1 through P4, there’s no P(TV Adapter). There’s only the TV Adapter “menu” setting as seen in the first screen shot, but there’s no TV Adapter “Program” page per se. The TV Adapter “Program” is just a handle added to allow you to cycle to it to receive streaming content from it.
The same principle applies to the ConnectClip. If connected to a ConnectClip, you’ll hear a “Remote Mic” program added to the P1 -> P2 -> P3 -> P4 -> TV Adapter -> RemoteMic -> (back to P1) cycle. But there’s no P(Connect Clip) in the second screen shot. There’s only the ConnectClip page per se.