Here is a suggestion for a workaround. If you know the document where variable A exists, then give that document a name in a multiple choice. Then, in your template use an "INCLUDE" statement for the document (template) and make the INCLUDE conditional with an if statement. Even if you can get the document name (I don't know if that is possible in HD Author) you would probably still use the above functionality except that your condition would depend on the name of the file rather than the name that you give it in your multiple choice.

ERASE does not functioning properly in my view. What is happening is that you DEFAULT a value, the default is ERASEd and then value is DEFAULTed again so it looks a a SET. ERASE should limit its functionality to the particular dialog in which it is used but it doesn't. It affects all. I have learned over time that ERASE is to be avoided at all costs.