This is a really odd argument because effectively Mike and tec believe the same thing, but neither can convince the other that they're right. Though I think Mike's wackier theories about "playoff resets" are dead wrong.
Mike: There is, you're right, a theoretical maximum number of innings a P can pitch throughout the year at 100%. This is determined widely by user input and the player's STA/DUR combo: number of pitches thrown in each appearance, if he is thrown at 100% as soon as he hits it each time, etc. We'll never know what that perfect number is probably because no one is perfect at minmaxing that.
That said, a P CAN throw "infinite" innings. If, hypothetically, the season was 250 games long he would continue to fatigue and recover at the same rate, forever, based off his STA/DUR. There is no tiered inning cap at which he starts recovering more slowly or something. I think that's fairly obvious. Even guys who make 40 starts have predictable recovery rates through the duration of the year. (Of course, as tec mentioned, not all 100%'s are created equal, but the rate of recovery between games remains standard).
Of course there is some number of innings the guy can throw in ST + RS but nobody cares about the number of ST innings, and as long as he is fully rested to start the RS, he has done no damage to the maximum number of innings he can throw during the RS.
So, if you want to make some P throw every pitch for your first 6 SP games, assuming the following 17-ish cycles are enough to get him back to a perfect 100%, he's not forfeitting any innings during the RS.