Okay, lets go step by step.
First, the point is companies didn't even believe he'd be able to make it, so this is why it is a hail mary for him, he doesn't even expect it to work in the first place (hence the happiness after he receives the mail). This is his last chance to succeed (remember he only has 300$ left on his bank account) so that's why he is all in on this project. And you know this happens in real life: really nice projects some guy decides are not worth it, or thinks are impossible to make or a waste of time, and end up not being developed because the middle man had short sight.
Second, about helping her niece be sluttier. Yes, he insinuated she might use her assets to get the job, but he assumes she'll be doing it in front of some "safe-lovely-old man" that needs someone to hire and prefers to have a nice view rather than a random guy working with him. When he does it he does not think about Mr Silver. At all.
Third: about bringing her to the meeting. There's an actual option not to bring her. When they get there, he tries to warn her also, but he fails. And I recommend you to play the prior scenes differently if you are worried that Mr Silver goes talking to her while you are busy, I don't want to spoil, but try.
As of his Uncle and main story, if you choose to bring her with you it means two things: the easy one, you don't care she comes or you are certain Mr Silver won't talk to her, as he is only attracted to certain women, or the second one, he does not want to bring her anywhere near Mr Silver, but knows that if he tells her to go home she'll be upset, as she'll feel she's being treated liked a kid.
And finally...
Where exactly are we selling our niece into sex slavery? x'D I mean, the whole point of not wanting to bring her to the meeting is to avoid that...
And I don't know about T4bbo, but I'm not Russian and I see the variables (the whole set of them, as the MC is who you decide it to be) perfectly logical.