What do you mean by "dead" path? If you mean "can't path dead things" there really is no Reason for her not to be able to beyond the Author saying so.
I'm under the impression that she
can path dead things once they actually show up (or, at least, can simulate them reliably enough), but cannot path/simulate
the Afterlife (and there's no reason why she
could, as her shard should not have direct access to it even for observation purposes), which in turn means that her path cannot (reliably) account for any visitors from there (at least, not until they actually arrive).
The result is similar to her canon problem with pathing new triggers, though more fundamental (i.e. she would still have had that problem even if her power was not otherwise restricted).
It's possible (depending on how the plot goes) that further integration of the real world and the Afterlife could result in Contessa's shard actually getting the required data for a proper Path simulation including it.
I doubt that this would happen, though; if nothing else, the Afterlife is probably too
big.
(Also, yes, I agree that Eden - or Scion for that matter - would have found the Afterlife to be exactly the kind of solution that the Entities were looking for in the first place.
I wonder what would happen if Scion somehow found out about this... on second thought, he'd probably insist on taking the Earths' population with him into paradise - i.e. killing them - anyway.)