New to Diamond. Chance of D1 upgrade

I'm on track to hit Diamond status with my next two flights scheduled for next week.

I have a 15 hour flight to South Africa in September and DESPERATELY want to take one of the 4 D1 seats left. Do they take into account when it is you hit Diamond when you add you to the D1 list? If there's other Diamond folks in PS, do they get priority over me in Main Cabin?

When I hit Diamond, do I automatically get to use a GUC to go to PS, then get added to D1?

I really want to spread out on the 15 hr nonstop flight. Can someone give me some suggestions on how to best execute this?