Bob, in my opinion (for what it's worth) in each of the scenarios there should not be A result but a range of results.
For example, equal teams + appropriate defensive formation + no variables = play gains between A and B yards (I don't think it fair the sim's run/pass call on defense significantly affect the result of the play unless pass coverage is set to short or long)
Equal teams + inappropriate defensive formation (ex.:nickel vs. wishbone or box) + play run to strength of offensive formation (run) + no variables = play gains between C and D yards
Changing LB assignments should affect the OL/DL equation, as the 'cover' and 'line' settings add or subtract personnel primarily stopping the run or defending the pass, in each case modifying the appropriate range of results. If an OLB is set to 'line', it would only make sense that it should be harder to break an outside run, again modifying the range of results. If all LB are set to line, the range of running results should reflect additional run stoppers. If all LB are set to cover, a running play's range of results should be modified to reflect all LB playing off the line of scrimmage.
The main driver of results, the initial setting of the ranges, should depend on talent. A three point differential between OL and DL should yield one range of possible results, while a ten point differential should substantially limit the potential for a lower end result and extend the possible higher end of the range (something like one percent chance of 0 yards gained versus twenty percent chance of gain of 10 yards or more). The talent level of LB, DB, QB, RB and WR would also have to factor into the equation - determining the appropriate range of results, weighting the calculation, and settling upon a result within the range.
I don't know if that's helpful or not.