I bet they won't.
If they can see which way the photon "flipped" FTL, then I think they'll be able to manipulate a message eventually
Begs the question. If they can see FTL, then they can certainly send FTL - you just have two people looking at each other with FTL portals: Give them each chalkboards. One guy writes something on his board, and the other one immediately sees it FTL through the portal. The other guy can then write a reply and the first sees it FTL.
But I believe the portal is impossible.