Certification should be required if someone wants to analyze and critique a methodology or is just not happy with it, to ensure they understand it. Obviously one can learn it itself but that seems frequently just reading a bit of manifesto and picking the pieces one sees fit for the purpose.

Was it fault of Waterfall that specifications and documentations are expected to be perfect before each next phase and are not subject to changes after they are done in one iteration, or it was a fault of teams taking things at face value from methodology and making it hard for themselves?

Agile is there to shift the thinking because people end up being busy more with the process than with the actual work. You have to deliver some product in limited time for business to survive so how do you best go about it? What do you find a better way of documenting that you think will help you and the team? Try it. Should this be a mandatory step before a development cycle is done? Up to you to see if that is the best way forward.

Some of the best products are result of what Agile promotes, even though you didn’t even need to follow Agile to do that. It’s just that people got so stuck in some way of working that they didn’t think what really makes sense. And no matter how much Agile emphasizes that inside the framework you find what works for you, people still get stuck in made up boundaries because of company issues. Mutual trust, understanding and adaption is the biggest requirement of Agile, and its biggest point of failure.

And your new methodology falls inside Agile. There is no extra crap. Those you mentioned have nothing to do with Agile. Even if your methodology had just one line: “do whatever you want” same companies would say, “oh, but doing whatever we want was a bad thing, but that is not our fault”.