Downwind_Left
Nice analysis. To give a different spin on it though. If Flybe/Flybe2.0 were to strike a deal with a major European airline there could be some tactical manipulation of these slots. Say KLM were to buy the slots, which as you say can only be used on European routes. Well KLM already have LHR slots for their AMS flights, so they use the new "Flybe" slots on the LHR-AMS route, satisfying the European requirement, and then they are free to sell their original slot to anyone they like, for instance a LH carrier who flies into Heathrow.
Of course this all depends on whether there are restrictions around the existing European slots that airlines like KLM/AF/LH etc already have.