Do all ticket categories (Bronze, Silver and Gold) automatically give me access to a seat in the grandstand?

No. SILVER and GOLD tickets give you access to the circuit (to the BRONZE area) as well as an allocated seat in a grandstand, whereas BRONZE tickets give you access to the circuit (to the BRONZE area) but not to an allocated seat in a grandstand.

Where can I buy the bus ticket for the trip Circuit Spa-Francorchamps to Verviers central station?

Tickets for shuttle bus users between Verviers-Central station and the GRAND PRIX site will be sold at a TEC ticket office located at the entrance to Verviers-Central station. The prices are as follows: Friday: 30 € (ticket valid for all 3 days) Saturday: 20 € (ticket valid for 2 days) Sunday: 10 € (ticket valid for 1 day)

On Fridays and Saturdays, can I go freely to other stands than the one for which I have a ticket?

No. Your grandstand ticket gives you access to your grandstand only, and to the seat you've been allocated. Seats are allocated for each day. However, each grandstand ticket also gives access to the entire Bronze zone.