Nowadays this is more practical using WSL to run a Portable TeX Live for Linux.
However due to the much lower cost of thumb drives that can carry both versions I would strongly recommend using a native portable Windows version & dedicated Linux portable version both on the same drive with a shared common Texmf. (You could even have both distros for both OS if so inclined)
There are a number of issues that we see through this site that lead me to this conclusion.
With reference to your "Side Question" about system fonts. This is actually a key problem. Every font has its own restrictions and in some cases it may NOT be legal to use a font even on the same single boot machine under dual OS. i.e. Windows supplied or installed fonts do not confer unfettered rights to use those fonts in Linux in Windows (WSL)
We cannot ignore such font licensing issues and without dwelling on specifics here (see other related threads if affected) this is easily avoided by keeping to "One System at a time" so if plug in a Windows OS MiKTeX or TeX Live use Windows structured fonts OR if using a Linux OS MiKTeX or TeX live use its structured fonts
If internet access is an issue and I know some rural users still have problems with large downloads then it is best to obtain a current copy of the TUG installation DVD which includes a Full TeX Live and ProTeX wrapped MiKTeX and export a portable copy of each to a usb drive.