Well, i can answer some of questions from what i know...
Iriminage does not really require 20 years to master, but it took O sensei 20 years to develop it. Different people have different ways of opinion about 'mastering' the technique, so it might require more than 20 years for some people, and for some people, less.
Irimi and tenkan are basic movements, but BOTH can be also said to be evasive movements, and then a technique to be followed up. For example, u can 'dodge' a punch with either. MOST techniques require such movements, but some do not.
If im not wrong some techniques require you to lead the aggressor to you, for example, throwing an aggressor when he comes from the rear with a bear hug, or when you lead an aggressor coming from the rear with a body lead and when he grabs you, you sink down. I don't really know their names... sorry about that. I don't think they would be called irimi or tenkan.
I think i can only answer so much, im only a white belter.