Work Permit Questions

So from what I understand, to the meet the requirements a player needs to be involved in 75% of his countries internationals...

A) Is this true?
Are they any other ways for someone to meet requirements (besides feeder club, and just sitting in training for a few years)?

pretty much everything you said. Some players are born with dual nationalities though so if one of the nationalities is the country you play in or your continent i.e. brazilian player but also has spanish nationality, that should negate the need for a work permit if you sign him for a european team.

Of course, i think the exception to the 75% rule is if the player was injured. If the player plays every international game when fit, then work permit is usually not an issue.

Do Under 21's and 19's count as International for work permits?

I do not think that it is 75% but it is easier if that player plays regularly for his country. Yes, youth levels would count.