Data is the result of science. You should go back to elementary science to learn the scientific method again. Overtraining came from eastern Bloc training and none of it applies to Mentzer's ideas. Do you even know who Prilepin, Issurin, and Bompa are?
Mentzer's idea on a set to failure and beyond becomes a muscular issue and not a CNS issue. You can tell you're not familiar with the CNS and its role in recovery and efficiency. This method taxes the muscles and connective tissue far before CNS fatigue becomes an issue unless you are horribly out of shape. Hmmm....that sounds like work capacity, doesn't it?
You point to X number of sets. Sets of what lift? Is their technique optimal? What's their level of conditioning? All these things are not accounted for in your philosophy.
I don't get how work capacity seems to fly over your head as it is completely correlated with recovery. This likely comes from your lack of formal education and understanding of the strength curve. You're even confused about the starting point; the cylinder begins at max capacity and not empty and, yes, the size of the "plug" does also increase. This is why it takes a longer deload period for the more advanced athlete to peak compared to a novice athlete. They know how to tax their CNS efficiently during the overreaching periods and start to taper volume as much as a month out from a competition. Conversely, a novice strength athlete may only need 5-7 days because they're not as efficient and need the skill practice.
Recovery on a small scale comes from improved enzymatic efficient at dissipating waste products intra-set. Lactic acid plays no role whatsoever as its dissipated within minutes (I really hope you don't think soreness is a result of lactic acid). On a large scale, it is the ability to replenish glycogen stores, bring the HR close resting levels, and keep nitrogen retention on the positive side.
You really, really, really need to stop looking at this as a BB'ing concept as it isn't. It's a strength concept.