iTunes 11 & macOS High Sierra & App Store issue

First of all, for a variety of reasons, I depend on iTunes 11, so there is no way around that. And it still works in High Sierra without any issues whatsoever.

However, the App Store stops working after a downgrade from iTunes 12 to iTunes 11. Apparently, iTunes and App Store are connected, and App Store won’t work unless you’re on version 12. It does work in Mavericks, of course, but any newer OS won’t fully support it by default.

My question is whether it’s possible to fix that?

During the downgrade process via Pacifist, I’m being asked whether or not I want to replace existing (“older”) iTunes files. Is it possible that some of the files being replaced are essential for App Store to work? In other words, if I’d choose to keep the installation files in question, is there a chance that App Store could still work as supposed to? And which files would that be exactly? Or does App Store actually require the full iTunes 12 package?

Any tips are appreciated. Thanks.