I have many horses and let me tell you, THEY SHIT. I have mounds 6 feet high of a horseshit/hay/sawdust mixture that when you dig into the warm, damp inner core it releases huge amounts of methane gas. If I were to buy a spore syringe of a strain of magic mushroom of my choice, would they grow and fruit each morning if injected into the core of the composting mass?

Ask on shroomery, and for fucks sake, learn how to grow mushrooms, you only need 1 syringe to to grow a shitload of mushrooms and they don't fruit over night, the highest risk you run is contamination.

Go to shroomery and ask them.

You can, but its not that simple. Do some reading on the shroomery.