OK, last page you mentioned 3 weeks old.
I will admit, I didn't read the first 3/4 of your thread, but I'll try and help.
I started feeding in MG moisture control, (which lot of people cry will burn) at 3 weeks from sprout, and I germ my seeds in the solo cups.
Your yellowing looks like a Cal. deficiency, but rather than try doing anything specific if you have not fed yet, (assuming your PH is good), if it was me, I would start a feeding schedule, starting with a light dose of the recommended amount for the first couple times, using a "feed, water, water, feed" schedule, then increase it after a week or so to the full dose and keep an eye on it.
Here's a couple charts that you can save, that may help you in the future.

PS, your curled edges look like it might be heat related, but again, I confess lack of reading.
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