Thanks!
Correctly written 
inline VBS code does, in fact, work, though (now I understand better how the system operates!) using an attached file does seem a bit tidier, particularly if/when you need to edit the code. It is easy to lose or misplace a ":" when typing in multiline functions on one line. What I think was the real key to my problem was revealed by your example - the Data Folder Property Name needs to be declared as an install parameter (as well as "filled in" in the code), then it is automatically handled correctly when you manipulate its contents via the Files and Folders page. I imagine that you could get everything to work without initially declaring the Property Name, but there are more opportunities to do things wrong that way, and I had.
Two more related questions:
1) In your example, you are using a single custom action in two places in the sequence. How do you do that? I find that anything I do in an attempt to produce that effect (copy and paste, write a new custom action identical to the existing one) results in the creation of a second, functionally identical, custom action with a _1 appended to the name.
2) In my scenario, there exists the possibility that there is more than one instance of a specific file at the same level in the AppData tree (but in different branches). Is there any way for a file search to return multiple paths, rather than the path to the first instance encountered in a (on the basis of my experiments) depth-first search?