As you say Matt, the only way that they could keep secrets like this would be to have a very small number of people in the know.
The henchman and secondary Bond girl stuff got out because of the casting search process. Agents, actors etc had to know and people involved talked.
The interesting thing with the main villain and Bond girl is that there hasn't even been any talk of them looking.
This all leads me to believe that they've had two actors in mind for a long time, they were approached and said "yes".
Little to no auditioning, no elaborate casting search.
We have to be looking at two well known and respected actors to bypass all that - people who Dan and Sam really want to work with.