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It sounds like you're about to reinvent [shudder] ActiveX.

Converting to JSON would be case of adding a new writer class that can write the yaml classes out - wouldn't be difficult. I will look at it when I have time.

I am not a fan of interfaces, but they have their shining moment, this is one of them, if you introduced C and only used by interfaces then compiler, versioning and implementation should not matter, it will work always. Just take your time to implement it future proof as much as you can think of, don't overengineer it, just keep it simple with multiple interfaces instead of one if possible, also use versioned (numbered) interfaces (in the name) to indicate to yourself and as reminder that these interfaces should not be touched/altered, if in the future need a feature then implement new interface and keep the older.

The vmt of all Delphi classes has vmautotable that is not used. I have patched a particular class vmt and in the autotable slot and stuck a reference to an object in there. So both of your units can check that reference on some class from the vcl that you pick. Then they can patch the class. I dont remember the patching code but if you cannot find it on the web let me know and I can dig it up.

A TPageControl is just a tab control with each page represented by a TTabSheet. If you look at the source of TPageControl.ChangeActivePage you can see that it sets Visible=True for the active page and Visible=False for the page being deactivated (if any). Since you can't paint a control that has Visible=False my guess is that what you are trying to do isn't possible without some really nasty hacks. What I think might be possible is to block repaints of the page control (LockWindowUpdate etc.), make each tab visible directly (via TTabSheet.Visible), paint them to a bitmap, restore everything, and resume repaint - or some variation of this.

You are missing passing TFormatSettings to the StrTo... functions that you are calling - that makes it fail on systems that use a comma as a decimal separator, for example.