(1) Regardless of wether I check 'use previews' or not, PP will render the timeline again. Regardless of wether the timeline is already rendered. Correct ?
Not correct.
If you check 'Use Previews', your export will 'Use Previews'.
With this box checked, and if your timeline is preview rendered,
the export will likely be faster, but will not be the best possible quality.
(2) By previewing, do you mean playback of your timeline ?
Yes.
