* Freediving is a personal journey, unlike scuba there is no real standard. However different organizations do have test to be considered a "basic" "intermediate" or "advanced" freediver. When I took the PFI course 11 years ago to get your "intermediate freediver" you had to hold your breath for 3 minutes and reach 60Ft (20meters) . Everybody who took the course was able to accomplish this. So...

** yes, I believe freediving is very easy to learn assuming: you know how to swim and are comfortable in the water/ocean, and you are able to equalize your ears. The breath holding is very easy to learn and teach.

*** The course is 4 days long. Beyond that you train on your own, but just swimming and playing underwater can be considered training. On my off season I roughly trained my breath holds bi monthly.

**** No, classes teach you how to freedive. I'm actually not sure what an "aggressive fish" is! Remember when your in the water with a tail or fins we are large, 9 ft long.... fish are naturally timid and it takes time in the water to gain their trust. I've even dove with sharks and it takes a lot of time to get them to come close enough I can touch them.