Thing one:
When I installed El Capitan it gave me iTunes 12.3.2. Under the heading “Things You Need” number three is a download link to iTunes 12.3.1 but that was for Yosemite. I don't think it would make a difference but i might have used 12.3.2 when I use pacifist to extract CoreFP.framework for iTunes 12 (
Step 5: All About CoreFP.Framework).
I never upgraded iTunes 12.3.2 and still have it hidden, as per instructions, as .iTunes (notice the dot). It is possible you might have gotten iTunes 12.6 with your install of El Capitan. Especially if you ran a bunch of updates before reverting and forgot to omit iTunes updates. If that is the case, maybe you need the CoreFP.framework from iTunes 12.6.
However, I think it shouldn’t matter and it is something else. Still, can you check which iTunes you hid with the dot method? I'd like to know.
To do that, start by setting up Terminal to use show/hide aliases:
https://aoxoa.co/show-hidden-files-on-mac-quickly-with-aliases/ It will make life easier for you. After you set your system to show hidden files, highlight the .iTunes and check your version:
(oh, make sure you don’t launch .iTunes when you highlight it. I click the app above it and use the down arrow on the keyboard to highlight .iTunes instead of clicking on it directly).
Thing two:
I think your issue has something to do with your problem duplicating the framework files. As you said, "changing the .framework extension to .framework2 broke the file association."
When you duplicate CoreFP.framework it will look like CoreFP copy.framework. When you rename it you need to delete “copy” AND the space in front of it, then add the 2 after framework.
Like this:
CoreFP.framework2
Not like this:
CoreFP .framework2 (notice the space?)
As you can see from the clip above, I have no problem duplicating and renaming the CoreFP.framework file.
If your CoreFP.framework file turns into a folder after renaming it, try this: Right click it and go to “Get Info” > in the “Name & Extension” field you want it to say “CoreFP.framework2.framework” Also, I do have the “Hide Extension” ticked.
Your fix could be as simple as making sure you have the correct CoreFP.frameworks in Privateframeworks folder.