Action scenes are always going to be a challenge for me -- much harder than futzing around with introspection -- but I practice them a lot, so it's more gratifying to get them right. :D
It's referenced in TLH. They go up there after Thalia saves them from the werewolves. Aeolus gives Leo the picture of the Argo II.
I think that Reyna, like Hylla, adapts to her surroundings pretty well. And of course, by the time we meet her, she's been the sole praetor of the legion for months, fighting off a coup attempt by Octavian, so that may well play into the coolness everyone senses.
I was thinking of the restrictions on Zeus's territory for Percy, but Poseidon, as I think of it, is not quite as petty, at least in the incarnations we see (though he's pretty changeable, and Odysseus might have a different notion of how chill he is). But there's something else going on as well.