that Miracle-Gro soil is OK
Im not disagreeing with you about the soil I havent tested it.
Ive seen nothing but trouble with MG soils and seedlings
a seedling thats "unhealthy... with smaller roots".... can easily be burnt even with trace amounts....given the necrotic areas now beginning to show I wouldnt rule out the soil yet

I see that the nitrogen content on the label , isnt "urea orgainc based" which is less likely to burn" its ammonical & nitrate which does burn plants.

Miracle Grow has just re-formulated thier Seedling Soil Mix within the past month , it now contains some of the Urea "organic type" nitrogen (0.10% urea nitrogen organic non burning type) the ammonical type nitrogen they coat for a slow release & less burn

maybe they old formula was burning too many seedlings, or why the change adding Organic non burning nitrogen

just my 2 cents