Nick,
Congratulations on your first spawn! Nothing like that first time feeling
Actually, I find myself in the same boat at the moment, as one of my other South American eartheaters spawned for the first time last night.
Anyway, as Alan mentioned, the eggs you found on the bottom of the tank were definitely dislodged from where they were laid. Being only 15 eggs, Im sure there are more of them with the male, so don't be surprised if you find another dislodged clump.
Since you have 1 male, all you need is the one spawning cave. I find the females like to sit under terracotta saucers or under a piece of driftwood.
You already have something that your male likes to spawn in, so there is no need to add anything else. Although another spot for the female who resides behind the heater might help her feel more at home
Eggs start hatching in around 5-6 days, do the eggs still look healthy now?
cheers
Andrew