My input is based on M17. First off, being a sim player vs cpu, I feel your pain. Having said that, working off the premise that
"we have to know what exactly the All Madden difficultly setting does" might not be necessary. After all, your goal appears to have the game play reflective of player ratings, yet feel like the game is pushing and challenging you, as would an experienced human player right? All I can do is cite examples regarding the steps I've taken to achieve this. Sliders, progression and traits need all be taken into account.
First off, I adjusted the sliders.
However, unlike most folks do, I defeat my user-sliders only on an even-keeled basis, while leaving the cpu's settings at default 50. The reason? To avoid unstable gameplay.
There are some outstanding player progression sliders on the slider section of these forums. While I didn't use them, I keep a watchful eye on mainly the QB's, to make sure completion percentages don't get out of hand. I hold myself to the same standard regarding my user QB.
Traits are also a big deal. There are folks who swear that EA invokes a "comeback AI". While I personally don't believe this to be true, I do firmly believe that the "clutch trait" is very important and may contribute to some people feeling this way, especially at the end of games as well as with post season play.
In closing, the biggest problem I had with All Pro on M17 was that the CPU simply didn't attack enough offensively. Since I tuned the game, if I don't switch up my coverages, change how I leverage my defensive backs, etc I will get my butt handed to me. The dice roll programming concerning the frequency of blocked kicks, an occasional play that feels scripted, etc I have not found a way to tune. Sliders work differently now where as you can't truly organically customize your settings, like you could in past generation editions.
Tuning takes a tremendous amount of patience, especially when playing a current year game and being subjected to patches that affect game difficulty. That was one of the main reasons I stuck with M17 for another year. However, with patience and perseverance, it can be done.