So, after having asked about how I would go about doing this, I finally sort of kind of got it to work.
So far, I have found mushrooms to be the most difficult thing to get into resin, and I was not prepared for it; whereas moss and pinecones and other found trinkets like that do not rot nor wither (moss is desiccation resistant, meaning it can never truly wilt), mushrooms, especially the delicate field mushrooms I was using, like to wither away to nothing in a few hours.
So after mixing the resin, before pouring anything into the mold, I had to first submerge the mushroom cap into the mixed resin, hold it down with the stirring stick just below the surface, blowing gently into the gills with a straw, trying to coax the air out of them. After getting enough of the air out of them that they weren't as aggressively buoyant, I put the now saturated mushrooms into the mold, trying not to trap too much air underneath them (a bit was inevitable), knock the caps off, or destroy the already distressed gills.
I poured about half the total volume of the mold in the mold, and then babysat the mushrooms a bit to make sure that they weren't touching the edges too much (if they press themselves against the edges, like they seem to like to do, the resin around that spot becomes too thin and will eventually break, leaving the mushroom exposed to rapidly decompose).
I waited about three hours - can't wait too long or the mushrooms will begin to wilt - before pouring the rest of the resin into the mold to cure completely.
I'm going to wait it out and see how they react after a few days, and make sure they don't still find a way to wither inside of the pendant, but for now at least, I'm pumped I even got them in there, after five attempts.