As far as I remember at the last Classic, the grandstand seats weren't roving, you had to stay in your allocated stand, however seats themselves were not allocated.
As far as I read the application form for tickets, like the last Classic, if you apply before a certain date, Grandstands are free.
Paddock tickets are a must as far as I'm concerned.
There was still plenty of space on MB on Friday, and as far as I know you can just walk into the site with your tent, no ticket required, except for general admittance. If there was a problem I am sure that someone from CA could run out in their car and get you through into the site.
Another girlie with me for their second Classic
Andrew, if you would like a copy of the ticket application form which explains prices etc, PM me your email address and I'll send you a copy of it.
My invoice includes ACO tribune seats for 4 people and Tribune Lagache for the rest.