Wow! So much good advice from so many! OK, let's see if I can answer all your queries... (1) One reason I was thinking of not carrying my 50-500 is mostly because of size and weight. It does take great shots, but it is a beast to carry, especially since I do not use a tripod or monopod in the safari vehicle. I have tried a monopod and it just seemed to get in my way. So hand-holding and using beanbag support or any other stable support I can find in the vehicle has done a good job. But it takes my arm and shoulder several days to get used to toting the 50-500. I may give up and take it, but was just interested in other options. The other reason, is that I have started seeing some dust inside my 50-500; inside the front of the lens. It has not yet affected my photos, but it is there and in a place where I can't get at it to clean it. I understand that happens no matter how much you try to protect your lenses while driving around in the dirt. These are the main reasons I was considering renting the 150-450, which is about the same weight as the 50-500 but about 1 inch shorter, and a newer lens, and would give me the zoom reach. (2) I LOVE my 70-200 F2.8, and have gotten so many crystal clear, sharp images from it, that I will be taking it, despite its semi-large size. Also, yes, I do have a need to photograph at night. On safari, we are out riding after dark as well as daytime, so the F2.8 is very helpful to me when riding around with the only light being a small spotlight from the guide. The 1.4 converter might be a good option for daytime use. Since you lose f-stops with converters, starting at 2.8 would at least give me more to work with. (3) I find from past trips that I have used the 18-250 more than any other lens. Very versatile, both for landscape shots, for that occasional animal who walks right up to the vehicle (we have had that happen many times), and for having at least a little bit (250 x 1.5) of zoom capability. And I do use my K3 for videos as well as stills. Sometimes you just HAVE to take video when there is a baby giraffe running around and playing

I always have two camera bodies, and try to have the right combination of lenses on each one so that I don't have to change lenses while outdoors. It's way too dusty to keep risking that. That's why I want to have the best coverage in two lenses.
That said, I will continue to consider the great advice you have all supplied. I have dealt with LensRental.com before, and will continue to do so. They were very helpful and very easy to work with. OOh, slyfox, lens breaking is indeed a disaster. On a trip to Yellowstone Park here in the U.S. I actually dropped my zoom lens over the edge of a cliff while carefully changing lenses (supposedly carefully, that is), and we ended up driving outside the park to a large city in Montana the next day and buying a new/used lens. That is something I hope never happens again!
Thank you everyone. I will check back to see if any additional thoughts occur to you, but I think this has helped me get my brain wrapped around the issue for now.
Grateful for you all.