"Industry debate" is a good term for it. The "no amendments at all" side are holding fast to their stand, but their numbers are declining. Hopefully, we can keep the "it's only allowed if it is specifically permitted" view from becoming "industry debate" in other areas.
1.401(k)-3(j)(1)(iii) has a definition of ACA that is used as part of the QACA definition. But, I don't see any rules in 1.401(k)-3 that a plain ACA must satisfy, so I don't see that the -3(e)(1) mid-year prohibition applies to an ACA. If the published guidance doesn't prohibit it, then it becomes a matter of opinion whether you can amend and the answer is determined by where you stand in the debate.
Bill, keep your fingers crossed that none of your clients die or sell their companies mid-year.