I haven't been on a gorilla hunt but have a lot of safari experience, so can offer a few comments on your questions. As in all things, circumstances are different and YMMV.
Eyepiece v. screen: I virtually only use the eyepiece, and make really sure that I've blocked any light leaks that might be around. In the jeeps, you have to stay pretty much in your seat (no standing or holding cameras way up in the air) in order not to startle the animals (who just see the jeep, not the passengers unless you make yourself evident.) So using things like the swivel screen (which I also have on a Nikon) just doesn't come up.
Shutter/aperture: I'll pass on this one; no one solution works universally. However, I find it important to use high ISO settings in low-light conditions mainly so that I get some depth of field leeway. I would absolutely recommend that you try out your camera/lens combination in numerous low-light situations to see which settings give you the best and most reliable autofocus, almost irrespective of shutter speed. Especially at long-ish zoom settings, I've found that a balky autofocus will ruin more images than any other element, and in low light - using either the eyepiece or screen - you can't tell how it's doing that quickly, so you have to trust the equipment. There's no point in freezing motion if it's out of focus to start. Assuming you have vibration reduction (either in the lens or the camera itself) then rely on it, but make sure your autofocus is as fast and reliable as you can make it, and choose your shutter/aperture settings accordingly.
If your camera has fast and reliable bracketing settings, USE THEM and bracket everything by +/- 1 or 2 f-stops or shutter speeds. Memory cards are cheap.
Beanbags, etc: Don't know, don't use 'em. Something really cheap would be my guess, or possibly a unipod instead (you can use it on the floor of the jeep as well as on the ground when you're on foot.)
Safaris are fantastic experiences; I'd practice, practice, practice with my gear before going so that it's intuitive and on autopilot in the field.