How can I make picture specific settings for a slideshow? In other words, how can I set parameters like e.g. Lightroom template, transition period, zooming/panning grade differently for each picture (or at least for subsets of pictures) in the same slide show?
It's the same answer that I gave you in your other question. The Lightroom slideshow is very basic, very simple. If you want to vary the length of the slides and use different transitions and pan and zoom, etc. then you will need a much more sophisticated program than Lightroom. Adobe Premiere Elements is one choice. And there are other good third-party programs designed to do what you are looking for.