[dice]

So, let me see if I am clear on this: you inoculated your
seedlings with one of the Mycogrow preparations, then
someone fertilized them with Neptune's Harvest, and after
that they ceased to thrive.

Neptune's Harvest does not qualify as a high phosphate
fertilizer. It is very mild-mannered, even at the full strength
recommended on the bottle.

What the fertilizer may have done, though, is combined with
something in the seed starting mix to become a binder, leading
to water running around it instead of through it when you water.

If you don't have a moisture meter, try pushing a chopstick
down through the container mix around one of the non-thriving
seedlings that looks and feels fairly dry. Does it penetrate
easily, or does it have to break through some kind of crust
before it will push down into the seed-starting mix in the pot?

If the seed-starting mix is forming a hard, dry crust as it dries,
that is likely the real problem. Early in the season, you could
have let them get fairly dry, then shaken them out and repotted.
Kind of close to plant-out for that now, though. I would just set
them in water, and let the plant soak it up until you can tell
by the weight that it has plenty of water.

Edit:

I did overgeneralize a little in the description of the interaction
of the Mycogrow Soluble spores with the soil and root system.
There are a few types of bacteria in there that do not require
a live root system to survive. But the mycorrhizae (fungi) all do,
and perhaps some of the other organisms in that ingredients
list thrive better on live roots than anywhere else as well (they
eat or otherwise use something exuded by the roots).