Is the topsort from "Structuring Depth-First Search Algorithms" guaranteed to be (reverse) stable?
Stripping the Haskell encryption from your question (and ignoring why you are using dff instead of dfs to get a search that respects a given order), you appear to be asking about the stability of the topological sorting algorithm that performs a depth first search and reverses a postorder traversal of the depth-first spanning forest. However, there is a more general answer to your question: it is not generally possible, given a sequence of vertices in a DAG, to find a topological ordering that respects the input sequence for all incomparable pairs of vertices in the DAG. That is, stable topological ordering is not possible.

To see this, consider a DAG with three vertices $a$, $b$, and $c$, and one edge $a\rightarrow b$, with the vertex ordering $bca$. The DAG has three topological orderings $abc$, $acb$, $cab$, but none of these are stable: $abc$ and $acb$ both reverse the pair $ca$, and $acb$ and $cab$ both reverse the pair $bc$.

For the same reason, reverse-stable topological ordering is also not possible. In particular the reverse-postorder-dfs algorithm is not reverse-stable.