Well the BKK flights leave from the main building but the MH lounge there is closed for renovation. If you have time you can take the train to the satellite and use either the MH or CX lounge there, I think the MH one is better.

The gates open about an hour before departure, and as security is at the gate that is when you can start going through. However, there is no need to go through right at the start and you can turn up a bit later if you want. There is no priority queue for security at the gate so be prepared to queue!