Now, back when you first posted on SC probabilities, in the dim dark ancient days of 2009 or so, lol, I thought much like you in terms of numbers and probabilities. However, as time went on, I came to the conclusion that all of that was pretty much secondary, practically irrelevant really, to what mattered about SCs. They are not a mechanism for regulating probability of success, nor of gating some valuable outcome behind a highly risky set of dice rolls, etc. Instead its a yardstick, really. Sure, it matters that you can either succeed or fail, and as a general proposition both outcomes should be reasonably likely to arise from time to time. That provides some sense of conflict and uncertainty, and lets the outcome be owned by everyone instead of being a product of any one specific set of rulings or tactics, etc.
After that I just went back to the original (1st errata) SC system, and mostly accepted the various tweaks that DMG2/RC added. However, I just run pretty much EVERYTHING as SC, unless its a combat, and make it very open-ended. Yes, a given sequence is likely to focus on certain skills, and I can sometimes say to myself ahead of time "oh, yeah, and if the player does X here, then I'll respond with Y, and that's likely to provoke a chance for the other player to use his character's best skill..." but I don't think the whole writing things out process that 4e always envisaged is that helpful. Most SCs are too organic, and arise too much out of something that the players start. It just provides structure, really.