when dealing with more than one timeline, how can I map a timeline of my choice to the player?
I have realized as a default always the uppermost timeline is mapped to the player window.
I have also realized that I can hide any timeline - also the uppermost - and then the next (non-hidden) timeline is mapped to.
Is there any easier method available (like just hitting the wanted timeline) to map it directly to the player?
That isn’t quite correct. If your uppermost track is 50% transparent then you see a mixture of the uppermost and the next uppermost track and so on. The player shows the sum of all tracks and filters, i.e. what your exported video will look like. All you can do is switch individual track visibility on/off using the “eye” symbol on the track.