Yes I know that you want to have the option in settings.
Still, you have not described why you need the context menu.
And you have not replied to any of the imminent problems.
Even in your use-case
It will not really work: what do you expect from the portable version of MP3tag in respect to the plausibility of the entry in the context menu? Esp. if you move to a new computer, then the setting would reside in the old settings which says "Context menu is enabled" - which it obviously isn't.
So, should MP3tag only enable the setting while it is running? Yes, it could be done to add a registry entry while MP3tag is running and remove it when MP3tag gets closed... but ... then you don't need the context menu as you can much easier drag&drop files from the explorer into MP3tag and you are not restricted to either open just one folder or just one file.
If you want to keep your settings, here is some advice in the HowTos on backup.
So please explain to me where in handling MP3tag this option would be nice without all the drawbacks of irritating setting displays and filenames pointing to Nirwana when the portable installation is not present - perhaps I overlook something.
Otherwise: these are the reasons (plus the general definition of portable programs that @LyricsLover linked for you) why you can't have the registration modification in a portable version.
Coming to your problem
To get all the setting right in MP3tag, simply install a new MP3tag into the folder where you have your old version with all the settings. The "new" installation will behave like an update installation and leave all configurations as they were. But if you choose "standard" as installation mode, then you also get the context menu (if you tick that option) plus the installation remembers where you put MP3tag.
When you call MP3tag, then the program looks for settings first in the installation folder and then in the usual Windows folders. So, MP3tag will start with the settings that you are used to.