References in a PhD thesis in mathematics
'Normal' is not really relevant. You must reference your sources. You must not artificially pad your reference section by citing things that aren't relevant. The question left then is how many standard texts etc you reference with regard to things like well-known background. This will probably depend on exactly what you study.
Yes, but it depends on field to some extent. In mathematics, references are almost always ordered alphabetically by first author's surname but check your university rules first, then a standard journal or three if that doesn't pin it down enough.
I'm sure this must have been answered before. If you quote something because someone else did, are you certain they were right? People make mistakes (and maybe occasionally deliberate false claims).