Ya know, I'd bet they'd sooner breed in a 20H than a 20L. I haven't tried yet, but I'm going to later this year. I just think that as far as smaller tanks go, the 20H is more conducive to the N. brichardi's habit of hovering to oversee territory. I know this idea is against the norm, but my observations point away from the standard "longer is better" rule with this particular fish. I did define the idea to a breeding pair, though. I've never seen any agression between a pair who breed regularly. I don't endorse it with groups or even pairs who haven't breed yet.
And the gravel? I recommend the gravel because I went through last summer and added about 80% more gravel to 4 of my african tanks. I observed a general increase in successfull breeding from all 4 tanks. My feeling is that they feel more secure in the deeper holes, even the mouth brooders.
The breeding progression in my 40 has been EXACTLY like yours, though I only have one surviving pair. Mean little buggers aint they?