I regularly inoculate my pots of plants growing inside with either culture or spore solution. It helps with growth immensely.
Seems like that primordia is dying off, maybe from RH levels....
Try putting like a plastic sheet or bag over the substrate and poking holes for fresh air exchange. I bet they will mature if you do that.
They look like just another LBM that one would find in their lawn after a good rain.
Wow! How is the rubber plant doing ? I wonder if it is a good symbiotic relationship or a parasitic one?
Try posting your photos and descriptions over at http://mushroomobserver.org - there are a great number of professional and amateur mycologists over there and there's a good chance somebody there will be able to figure out what you are growing.
Found it - it's a Lemon Yellow Lepiota, Leucocoprinus birnbaumii, also known as a Yellow-Pleated Parasol.
Those look great any chance your going to take some sporeprints?