Am I reading 1.414.(s)-1(d)(3)(vi) and 1.414(s)-1(f)(1) correctly - you cannot use a compensation definition that includes prior-employer compensation in a defined contribution plan, only a defined benefit plan? And if so, what's the logic?

The logic is they are 2 completely different types of qualified plans. It would be like saying, "My car requires gas to allow it to run, but my toaster does not.".