Considering it's a community setup, I doubt I'll ever actually see offspring unless the parents are crazily diligent. Not that I mind though, don't have any experience raising fry of that size.
And I think the BN is just attached to the rock out of familiarity. Before he discovered it he would always hang underneath an upturned piece of driftwood, except he was literally right in front of the main glass wall in full view of everything, which is likely why he started to burrow under things out of desperation to hide more effectively and by the time I added the log he was already comfortable as is.
What I find weird is the female BN doesn't really care about hiding, she's quite active and is constantly moving around throughout the tank no matter the time of day, always side by side with her snail and cory companions (one of these days I need to try and get a pic of her sharing a pellet with a snail, it's the cutest thing).