Okay, so you're writing an independent game that will utilize revised mechanics from D&D, but will not require D&D to play. Okay, so far it resembles other games that function likewise and contain the OGL.
I supposed that you can change enough to call it your own, but that doesn't appear to be what you want. You appear to be saying that these are not mechanics that you can call your own, which would suggest that there may be an issue.
You state that the mechanics cannot be protected according to your interpretation of the law, but how far does that extend? A mechanic in which the roll of a die (or dice), modified by skill level (and perhaps other things) that must meet or exceed a value in order to indicate success would not appear to be an issue. Many games do this.
If, on the other hand, the specific die is a d20 and the skill level is purchased with skill points gained at certain character levels and measured in ranks and modified by attributes would start to get somewhat more shaky. You add in movement rules, armor, weapons, magic system (be careful that you don't step on any magic systems from other companies that are protected), saving throws, etc. and you're walking onto even shakier ground.
If you take some of the base ideas and truly divorce them from the d20 system, you may very well be legal. It just isn't the impression that you are giving to people.