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.

Which bus do I need to take from Verviers central station to get to the circuit? How much does it cost?

Bus routes 294 and 395 leave from Verviers-Centrale station and travel to the Spa-Francorchamps circuit EXCEPT on Sundays when they are diverted and DO NOT serve the circuit.- Tickets cost €3.50 per journey on these two public lines.- TEC season tickets are only valid on these 2 bus routes on Saturdays and Sundays and not on Shuttle services.