I haven't a clue how to deal with the technical aspects of this question, but I
have a suggestion anyhow!
This is exactly the sort of real world practical problem that needs to be
written up and posted in a *very* easy to find place. I think there should be
a faq, or a case studies page or something similar at spf.pobox.com where
problems and resolutions similar to this are presented. We stear people to
spf.pobox.com as the primary resource on spf. This is where case studies
should go.
I wish I were qualified to start writing such a page, but I am not.
Whoever builds such a page, and does a good job of it, will do more for SPF
than nearly anybody else.
Mark Holm