If you want to have it in the path just add it in the path !
If you use TC from a single location then it is better to
add your batch folder in the path system wide. I can advise you to use
RapidEE for environment variable edition.
The variable
commander_path is created once total commander is launched. So if you want to use it, you will have to set your path from TC process or sub processes. To do it from TC only once, IMO the best is to install
AutoRun plugin and alter the PATH variable as you described using the ability to set env variable from autorun. The installation of autorun is a bit more complicated than usual content plugin. Just read and follow instructions of the help file.