is it possible to always start with survey 1, but that surveys 2 and 3 appear randomly for each person that answers? If yes, how?
Yes, with a custom redirect script. (PHP/Script)
as the surveys might feel a bit long (at least to some), is it possible to create pages of, say, 10 questions each? If yes, how?
What about the progress bar?
would it be adviseable to merge the surveys into one big survey and create the pages (groups of questions as stated above) within this big unique survey? If yes, how?
Mergin automatically ? NO. You could import the question groups into the parent survey.
is it, conversely, possible to split a survey into, say, two (or more), separate surveys? If yes, how?
No
is it possible to extract the data, like .CSV, in order to analyse them in a statistics application (RStudio, Statistica, etc.)? If yes, how?
Yes
You should play around with the LS Demo:
www.limesurvey.org/de/demo
Cheers Kai