This is actually a long comment. Concerning point 2: Here is an amusing example when the referee wanted to collaborate with the author on the paper that he/she was refereeing, and so contributed a two-page appendix to the paper without disclosing their identity:

Real reductive Cayley groups of rank 1 and 2. With appendices by Dolgachev and an anonymous referee. J. Algebra 436 (2015), 35–60.

(Excuse me for promoting my own work...)