ultimately, one of the problems trying to answer a question.
you answer it one way and then realize later the question pertained to slightly different conditions. (missing the point everyone in their own group)
the IRS has said that if everyone is in their own group you can't use avg ben test for coverage. And the proposed regs wanted to apply that to nondiscrim testing as well, but they rescinded that item.
I think most agree excluding terminees is a reasonable classification (I think even the regs imply this when they say you can do this but if they work over 500 treat them as includablle and not benefiting i, but irrelevant once you have everyone in their own group.