Actually miracle grow is not half bad as a foliar feed... the salts are worst when allowed to build up in the soil. On the leaves that is not an issue at all.
Fungicide, anything that contains Bacillus subtilis... garden shops SHOULD have something.
Potting deeper than the 1st set of leaves IS a problem. If that were my plant I'd be digging down to see what sort of shape the stem was in below the soil surface, and if there were no roots, actually removing some of the top of the soil. I'm not going to suggest you do that for fear of damage to the plant, but curiosity ALWAYS kills this cat, lol, I guess it's up to you...

Okay... extreme first aid for a plant that I suspect has rot under the soil surface... disclaimer!!!! This is how I do things and may be totally wrong!!! I have had this happen a few times and DID lose one, that was rotted through the main stem, but saved others, it even worked on my Christmas cactus that got rot.
Repot- the soil it goes into should be EVEN LIGHTER than you would normally use, like if your normal mix has 1/3 perlite, go up to 1/2 or more perlite. You want the soil to be able to dry out FAST. Pot up in a larger pot. In fact, look up the term 'elevated transplanting' this is when you repot, but don't settle the root ball so the soil surface is even wiht the new soil, but a couple inches higher, for airflow around the whole thing.
Drench- use the bacillus subtilis as a soil drench. This is where your nice light soil is great, it will flow out fast.
Keep out of strong sunlight- the roots are not functioning at full efficiency so the plant can't uptake water as fast as it needs to if it is in strong light.