Couple things:
1) You won't see animals WHILE you're actually eating. Boma's windows look out at the pool area, not one of the savannah's. While you're there, though, you can visit one of the public viewing areas (Arusha rock, couple of spots out by the pool, etc) to view the animals. Given it'll be late september, you might want to arrive early and look around. Then, after you eat, if it's still light you can go out and look some more.
2) As far as I know, the prime rib is ALWAYS on the buffet. We've eaten there numerous times, different nights of the week, and it's always there. I've likewise heard from other guests the exact same thing. Nobody I've spoken too, in depth, about eating there has evern NOT had the prime rib on the buffet.