A halved coconut may be taken as a cave, – so it sounds like this is what is happening.
Are there other caves available ?
Because I would say its not the ideal cave, but then again, – in nature the fish will sometimes have nests underneath leaves, or other less structured things, – so its not always that there a fine little perfect cave available.
In a tank though, I think this kind of cave is not ideal because the fry will probably tend to leave earlier than in a small narrow cave, so the male wont be able to look after them as long time.
I have generally found that paros simply prefer at all times to stand underneath something, – so if theres somewhere which is like a shelter they will seek underneath that. I have a tank with a big ketapang leave which has fallen down and all the paros stand under this – not nessesary for nesting but simply because they are ‘protected’ from being seen from above.
I have also found that in tank with less floating plants the paros are less seen than in those with a thick layer of floating plants in the surface, so they prefer to be ‘under’ something.
Your second question about how long the bintan forms defend their nests … I dont think theres much difference to other species. My experience is that the male will take some place in the tank as his preferred ‘home’ base, and he will stay around there whether he has eggs or not. The actual time in which eggs are developing is around 10-12 days, and if the fry stay in the cave all this time the male will also be in the cave and strongly defend it. But I have experienced that in big caves, such as probably a coconut, – the fry will leave earlier, and once gone the male does of course not defend the nest in the same way.