Every time I start torbrowser the about:tor page opens.
Using what method?
When using /home/user/tor-browser_en-US/start-tor-browser directly, in Whonix 9, that is stock behavior in any case. That means, the same would happen on plain Debian. Then https://www.whonix.org/wiki/About#Based_on_Debian applies. If you reproduce it on plain Debian, you get the ability to ask support questions and feature requests to The Tor Project.
The desktop / start menu icons have been added by Whonix.
What page to auto open is currently not configureable, but changing LINK=“about:blank” to something else should be not be that hard. I can make that configureable for Whonix 10 through an /etc/torbrowser.d config snippet. No big effort.
and because of some custom configuration, I am always told that 'tor isn't working'.
This can be a bad sign for anonymity by the way.
If possible I do wish to retain it for my main torbrowser, but for my other custom config torbrowsers I don't want it happening. Can someone explain how to disable it?
I am not sure what you're asking. You are using two parallel installations of Tor Browser in different folders? Please see also my signature about effective bug reports.