bear with as I am typing from my phone. I DEFINITELY sympathize with you. I honestly believe this train of thought you are following leads one of two ways: a) the person puts a really good amount of thought into what he/she is doing and learns a TON, making mistakes along the way but moving forward and eventually reaching veteren status and their goals.....
Or b) they get lost in analysis paralysis and forget the first rule to success is busting gut wrenching ass 100% at all times. Singleness of purpose, as you so succinctly put it. Singleness of purpose drives everything, at all times. Not just in fitness but everything: basketball: MJ. business: Gates, buffet. War: ancient sparta. Physics: Einstein. Powerlifting: louie simmons. Bodybuilding: Arnold.
If you couple singleness of purpose with critical thought and creativity, you get a monster in the best sense of the word.
Mistakes in training too little or too much are bound to happen, it is what you take away from them that matters most. Same with diet. A very smart man once said "martial arts is like a suit. I wear a suit. It's tailored to fit me, to my measurement and my movements. a lot of teachers try to take their suit and give it to you. But you are not built the same as them, so it won't fit. Instead, try to learn how to measure for yourself. That is what I am trying to do" paraphrasing obviously. Same goes with fitness.
Put it this way--philosophy is good. Thought is good. Necessary, interesting in its own right even. But it only really comes into it's own with action. The ONGOING STRUGGLE in 100% action is singleness of purpose. Ecerything else is secondary, however useful it might be. You never learn how to play a game sitting in the stands watching. You may get conflicting advice from your coaches but you will never know what to do until you take action yourself and try one of the coaches advice.
In all that you read on this site and many others, there is one CRITICAL question to bear in mind while reading--ask yourself what the target audience is. What's their background and likely problems? That will help you sift thru these things, because articles have to be targeted to somebody.
Couple singleness of purpose (100% committed to the struggle style action) with self-education/critical thought, and creativity. But never EVER lose that singleness of purpose. That is what everything else builds off of, even mistakes made and learned.
Without it, you have nothing. Keep hustling!