If there was an insulin response [to Diet Coke], but no ingested glucose, shouldn't I get hypoglycemic ?

You don't think it's at all possible to over-produce insulin without the resultant hypoglycemia? Seems to me the insulin resistant do it all the time.

I think the more relevant question (at least as far as fasting goes) is, if you did get hypoglycemic from a Diet Coke, and you were in a state of ketosis, would you necessarily even notice?

But what other potential effect might that resultant insulin spike have? And that's my view; if there's even a 10% chance that I'm jacking my metabolism by consuming sweeteners, then the sweeteners aren't worth it. I'm certainly no purist, but my next fast will be water and bone broth only.