Different people learn differently. There are things you will learn in-game that some other players won't catch unless they watch the replay. And things that you won't learn by playing the game. There are certain mistakes that you could make 100 games in a row without ever having a clue. Maybe for some concepts, the only way for you to learn it is if someone else just tells you. A one-size-fits-all approach doesn't work because people are different from each other. If you are benefiting from watching replays, then keep it up. If you think it's a waste of time, there are other things you can do.
Some chess players can recite their games from memory. They get out of a game and they could tell you a complete list of the moves both sides made. Other people can't do that. Someone who can't do that would benefit a lot more from watching a "replay" of their game, than someone who already accurately remembers everything that happened.
I definitely agree with the approach of picking 1-3 things to work on improving at a time. Zock himself said to pick just 1 thing, but that every time you go into a match, you always have 1 thing you want to test or improve.
And keep an open mind. It really doesn't hurt for ZappaZapper to think T3 land is silly and T1 spam is best. Sooner or later he will learn the value of moving up to T3 land. As long as he keeps an open mind, when his opponents eventually show him the value of T3 land, he can learn and adapt. In a sense, everything you learn from playing against 500-rated players is dubious. If you only watched games where FLOOD rolled 600-rated noobs, you would learn the wrong lessons about what works and what doesn't work.
If Blackheart or Thomas wants to take ZappaZapper under his wing and play 1v1s against him for 6 hours a day and explain the right way to do things and the right way to understand the game, that would be the best way to get Zappa up to 1900 as fast as possible. Anything else and he will develop bad habits on the way up the ladder. It's basically inevitable that people will get bad ideas and bad habits as they move up and part of moving up is not just adding things, but also discarding things.