As a question to the European pilots here, do cadets in Ab Initio programs need a prior degree or do the airlines track their progress throughout training to see if he or she will be a decent fit for them?
No, they do not. Some have a degree(s) but it's not requirement. Pre-course selection is used to identify candidates thought suitable for the employer, although student's are assessed throughout training in terms of technical competency of course. Unusual but occasionally the odd one or two don't make it through type-rating once exposed to the operators own trainers.