If you have a jeweler connection (did you say cousin?), I'd make every effort to put that to use. The markup on diamonds (especially high-quality) is ASTRONOMICAL. I used a long-time family friend when shopping and saved about 40-50%. Certification is a way of "guaranteeing" your stone and could be valuable if you were to ever trade-up or sell.
As for the wedding/reception talk. . . .unless you're made of money, I think there is a lot of sense in the points made above. Have a modest wedding and reception, give people a chance to wish you well, then blow the big bucks on a real nice honeymoon that you'll never forget! (Think Hawaii. . . .)