What narrative reason is there for Lena to indulge Jack? What does Jack offer that can't be found elsewhere in the story?
No reason at all, besides Jack being a hot big guy VIP flirting with her. And you don't get the option if Lena and Ian are a couple unless she's already cheating on him. So yes, it's just for the fanservice and for the porn. Or at least I hope so, because it would be worse if Jack actually has an impact on the plot. The last thing this game needs is another open path.
What reason is there for Robert to exist as a LI?, because fucking a waiter to maybe help to keep your waitress job has to be one of the saddest plots ever. And Lena liking Robert for real, it's maybe even sadder. But this is an AVN and there has to be plenty of sex, so here we are.