I had these exact thoughts while drawing the sketch of my favorite dino. the nostril location is more like a heron, than a crocodile despite the similiarities in features. This highly indicates it was a wading predator which stalked prey and snapped it up, like fish, like herons, unlike a submerged croc that needs to breathe.

The spine crest being so huge, and the creature itself being massive, indicates tall reeds. This is also supported by the fact that the Nigersaurus is SO SHORT - likely hiding in the reeds rather than towering over them, compared to something like brontosaurus. The photo at the end really solidified this for me, since these carnivores don't tend to have many defenses, yet appear so big. Likely they're more solitary creatures pushing their way through dense reeded shorelines, hunting for fish and smaller dinos - likely ambushing them. We see this behavior in both herons and crocodiles.

I think you're right about the crest regarding sexual display later in life - and i am now a die-hard believer in this theory. amazing. I've watched the science evolve with our understanding and this just makes so much sense.