1. I don't think it's too complicated.

2. I don't understand your question. All arriving passengers arrive to the wide open terminal. From there, a passenger either connects to a departing flight or goes downstairs to pass through immigration and enter Panama. Aircraft arriving to non-jetbridge landing locations or jetbridge spots are treated the same: i.e., bused to the terminal and released to an open terminal area. No passengers are directed directly to the immigration area.

3. Yes, customs forms are required for all persons entering Panama, regardless of declarations. If you are connecting, ignore any and all forms; you need none.