Wow, weird. So, I didn't quite understand, in the end did none of the fish die, just sort of pass out? Or did you suffer losses here.
I'm thinking in a sort of "unexplained" tank crash or near-miss, it might be a combination of variables that just happened to line up. Like, probably, it WAS a low O2 issue, but exacerbated by a spawn earlier. I had a clam spawn once do a darn near tank wipeout, just all of a sudden fish started dropping out of the sky - there was no indication in the tank although the sump turned out to be a mess of goo everywhere.
Also had a large H. magnifica anemone for close to a decade - thing was huge - if he so much as burped or got ticked off for any reason, fish would die. In fact it was the tank wipeouts that pushed me over the edge with that thing and I got rid of it. Sadly, it died a month after being sold because it cratered the buyers tank.
Such a shame it had to end that way. That thing would spawn too from time to time and man what a mess it would make.
You think about the things that can spawn in your tank - urchins, snails, corals, even fish sometimes .. it can put an unpredictable load on your tank out of the blue and for the most part it just filters out and may just be perceived as a slight bump in nitrates that even them in the end just disappear a week or so later - but then you add the proverbial straw to the camel's back by turning off all flow for 20 minutes and poof, it's just enough to push things over the edge.
Not saying for sure this specifically was it, but it could be just an unfortunate lining up of 2 or more variables at the wrong time.
I'm not sure I would be turning off my sump return when I feed, I've never done this myself as I don't like the idea. I do however turn off all the controllable streams when I feed. My two tanks are fed off the same wavemaker, my FOWLR does in fact go silent other than the sump return, but my big reef has a 6065 or something (I forget the model, a smallish noncontrollable stream) that stays on near the surface. This has the added benefit that food added gets immediately dispersed the length of the tank which gives the smaller or less aggressive fish a better chance of getting a fair share of feed - otherwise the bigger faster fish get the lion's share.
Anyhow something to consider, if I were you, I'd keep the sump return on when feeding, especially if it's going to be longer than 5 minutes for quiet time.
Good luck and sorry for your stress. There's always something in this hobby..