Alternatively, you could change the home page to the program's interface e.g. 127.0.0.1:7657 for I2P and start the browser with a script that creates a popup box using zenity or similar that tells the user the information.

Please reference

I don't know about running multiple Tor Browser's in parallel, but as
far as this environment variable is concerned, there won't be an issue
most likely.

The starter script of the alternative TBB instance could overwrite this

global, default environment variable.

The starter script of the alternative TBB instance could overwrite this
global, default environment variable.

I realized the homepage was set by an environment variable.

Thanks! Merged. Suffices for Whonix 11.

I'm all finished now. Didn't require much at all.

Good question. I don't really know for sure.

Shouldn't take very much at all.

How much work is that?