A high EC, especially if it’s N heavy, won’t express as yellowing leaves but the opposite with deep green leaves, curled leaf margins and bright green new growth. I wouldn’t expect to see nutrient lock out in only 15 days unless you’ve been blasting them with nutes right off the bat. It’s possible it’s completely unrelated to the nutes and was simply a result of being in the dark after Fiona. It’s always good to keep in mind with soil that it takes a bit for the plants to show the effects of what’s changed, they aren’t overly reactive like we are. Which is hard as we all tend to pamper our wee ones and fuss over them like mother hens.
With that said I wouldn’t be running my EC higher then 0.5 (or 500 millisiemiens but I’m just not use to seeing it that way lol) for seedlings anyway so everything you’ve done makes sense, flush the soil to get the EC into a range it should be at and give them time to show you how they react.
This is also why I take detailed notes when I’m using a new system or ferts, it’s much easier to learn if you have detailed notes of exactly what you did and when along with pics. Especially for dialing in optimal feeding levels for a clone, if you have data you can look back over a couple crops and notice patterns like yellowing lower leaves at week X, which lets you know the plant needs the N cranked up a week or two earlier.
Glad you and the babies weathered the storm as well as can be expected and hope you all get some rest and recovery.