Victor: Let me weight in here if you don't mind. You gave me quite a fright, as this is a feature I will be relying on - so I did a quick test, and think I found what is happening, and how to correct it.
First: As much as I love ADDT Includes I find the WA Plug-ins are actually better. Here is why!
ADDT Includes always included the file at it's "include position". The head was never extracted and placed at the head of the main document. Therefore queries were being run from within the body, not within the head. New Document declarations were beign added half way down the body - etc... etc... Pretty ugly when you view source, but it seemed to work.
WA did it right in my opinion! They include the file at the very top of the page, then just insert the body at the place you want it to show. THIS IS MUCH BETTER because the head data will actually be included in the head!
So why doesn't your variable work?
Simple. The file is being included, parsed and stored before your variable is set! Make sure the WA PlugIns are done AFTER your variable - and everything will be fine. PlugIns are added to the top of the page - so you will need to move them down to a location below your variable. Or course, if your variable is calculated inside the body of A, that could be a problem, you may need to re-think a couple of things and define all your variables earlier. It's all in the load order. Load your variable before the page is included and the page will inherit it. (remember, the display of the body is NOT where the page is loaded. The load is at the top)
ADDT always seemed to get the variable, only because it always loaded really late.
Personally - I prefer this system and look forward to re-thinking a couple of things to make it work. It will be cleaner in the end.