The way I think about it is that you can tell if a past participle is being used as part of the main verb phrase if you can transform the sentence into an active one and keep the meaning. If you can't then, it's a participial adjective. I've not seen this written down anywhere so please consider this theory as tentative.
a) Bad weather has never delayed my journey. ("due to" has the function of "by" here.) - verb
c) They sometimes cancel flights - verb
b) is hard for me to give a definite answer. The meaning of "get paid" is the same as "be paid". So : "people (employers) should pay people (employees) for days off.." This accurately transforms the sentence but without using the word "get".
As the phrase "to get paid" is idiomatic and colloquial, and "to be got(ten) paid" is never said, it may be that "paid" is defined as an an adjective, and this would be my guess.
It's obviously more important to know how to use the phrase correctly than to classify it, though correct classification does help when analysing language. Good question.