A topo sort is not strict enough for what you want (it just guarantees parents are returned before children), and in this example returns (A, B, C, D, b, c, d, e, E). Is there any way to force an alphabetical order on your vertices? I'd try adding a fixed length prefix to every vertex as you're adding edges, if at all possible. E.g., start with $p = "0000" (or however many places necessary), and just do $p++ before adding vertex "${p}_$v".
And then strip the prefix as each vertex is returned before you need to use it.
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