JRPass

JR Pass Green confusion

To try and be brief, I am currently in Japan, and my family in the US is mailing me a JR Green Pass (I personally would have opted for the regular, but I can't complain).

I am currently in Fukuoka (Kyushu), and plan to take a trip from Hakata up to Tokyo.

First, I would like to ask if I can exchange my voucher for the 7-day pass PRIOR to the first "day of use" without eating into those 7 days. I ask because I am told you can't reserve seats until you have the actual JR pass. I would obviously like to reserve a seat ASAP, as opposed to trying to make a reservation on the day of my departure.

Second, I'm still trying to figure out how the rail system works. I'm not even sure which line(s) I'll need to take to get to Tokyo. However, since I will have the Green car pass, I'd like to make full use of it by riding in a green car...even if it means waiting a few hours or departing a bit earlier.

I've been reading, though, that Kyushu shinkansen don't have green cars...? Did my family waste their money on the green pass, or is there something I'm missing? I'm still at the point where any train schedules I can find are baffling.

Please help! Thank you in advance.