Welcome to the club.
1.) if I can configure a 3CH version to only use the front and rear cameras and ignore the indoor camera - even if it is connected.
No, you simply pull out the connector just a bit for the IR Interior camera.
This is very easy to do since it’s a 3.5mm jack plug.
2.) whether in this case (i.e. 3CH version is configured as in #1) the 3CH version would produce just as high bit rates as the 2-channel version does.
In 1-CH the Front camera Bitrate is 60 Mbps.
In 2-CH REAR the Front camera Bitrate is 60 Mbps.
In 2-CH IR INTERIOR the Front camera Bitrate is 36 Mbps.
In 3-CH the Front camera Bitrate is 36 Mbps.
In other words connecting the REAR camera does not lower the Front camera’s Bitrate.
Only connecting the IR Interior camera lowers the Front camera’s Bitrate.
I still recommend you buy the 3-CH because in parking mode all channels Bitrate will drop to 4 Mbps.
You will get the benefit of the IR Interior’s POV looking out your side windows.
During normal driving you can disconnect the IR Interior to achieve 60 Mbps Bitrate on the front camera, and when you park re-connect the IR Interior camera.
Also the 3-CH version is only $20 additional cost.
If you buy the IR Interior camera separate later it will cost $70.
Here is what the IR Interior POV looks like mounted on the rear window.
Take note of how much you can see out the side windows;