I mentioned that calling, for example, delayMicroseconds(10) => 10 Times would sometimes only delay about 3 microseconds.
I didn't see that in the post. I looked again and I still don't see it...
You should disable interrupts around the actual delay; an unlucky interrupt might take a couple of us, and cause timer overflow when you weren't expecting it.
Wait; are your results all from your test program, or from other attempts as well. AFAIK, the problem I noticed should only happen when calling delayMicroseconds() with a constant argument; your example where you constantly vary the time in a loop should behave differently (and less "wrong.") Hmm. I guess I should look at things more closely!