Accelerate interviews (by a few months)? Please advise

I recently talked to a senior professional at a firm I am really interested in. He got back to me and said they're interested in hiring, but not for another 6 months for my level.

In the meantime, I'm interviewing with a few other places. One's with an investment bank for IB lateral analyst role. It's getting serious (probably the last round) and on the possibility I get the offer, I still have interviews with other places I would prefer (but have yet to advance as far into the process).

So 2 things: 1) Should I respond to the senior guy at the firm I'm interested in? Do I just tell him my situation (I'm going through a few interviews, but they are a top choice)? I can't see how they would move the process up 5-6 mths... would this cause them to think 'well, he's got other options, I guess he's not it'?

2) I know I'm counting my eggs before they hatch, but if hypothetically I get the offer to the IB lateral job, how do I politely ask them to give me time to think about the offer without them taking it away? And generally, how much time would they give you for a lateral analyst consideration?

Thanks everyone.