You have a Thinkpad? As far as i have heard their linux compatibility is fairly good.
An yes, ubuntu won't have more problems than windows with low power devices.
My old 4GB ram, 1.4GHz 4 core laptop ran linux mint fine.
Ps: linux mint is a ubuntu fork, and has a desktop interface that looks a lot like windows (example: in the bottom left there is the linux mint logo that you can click to open the start menu like on win 7).
If you have a legitimite minecraft account, you can use multimc (and probably prism) to launch a game in offline mode (and choose a custom name). This works for example if you want to play with a friend in a lan world.
I can also do it with my right hand, but i did it with my left first cause there is this german phrase "doing something with left" which means something is so easy for you, you don't even need your dominant hand for it.
You can just not git add the one file on the secondary drive when it is not connected.
This schould not be that much overhead and (at least for me) it is much simpler than any other solution.
Ps: Why do you want that one config file on the external drive and not on the main one?
In c single quotes are for single chars only, while int is a string. That means you would need " around it. I think.