It has been my experience that 2 rifles is best and BigUglyMan's second scope idea is also mandatory IMHO. I always carry a second scope in mounts ready to slip on, check and go.
I have found that you are not carrying the wrong rifle because you generally hunt different areas for different game. You don't often run into buff when hunting plains game and when you do go for buff you got your heavy and aren't hunting plains game. If having the wrong gun on foot concerns you you can always have one of the trackers carry your second rifle. PH's also hunt a certain way and you will find they take you out for antelopes first, no where near anything big, to see if you can shoot. So you will bang away at antelopes for a few days until the PH is happy with your shooting and you are used to the heat and no longer jet lagged. Then you will load up and go specifically for buff or eles or hippo. Lion and leopard in SA are baited, so you sit in the blind and hope, and hope, and hope.........416 for this.
Last time I went to SA was just a couple years ago and there was no charge per rifle for importation (might have been 10 or 20 bucks). I did pay Air 2000 to do all the paper work and get my permits (they specialize in this), I think it was $150.00. If you were going espescially for BIG game and throwing in a couple antelopes one rifle works great but if you're taking 15-20 antelopes and a couple heavy, dangerous critters I highly recommend 2 rifles. Your 300WM and your 416 Rigby is a perfect combination.
Caramel, if something catastrophic happens to one of your rifles, don't fear your PH will have another rifle for you in less than 24 hrs, and quite likely will have 3 or more in camp at your disposal if needed.
Planning for all eventualities is a good idea, but remember these guys aren't called Professional Hunters for no reason. You can even call ahead and they will purchase ammo for you in case yours doesn't make it, or in the case of the Rigby and 300 Win might even have some on hand already. There is nothing you can buy in Montreal or Toronto that you can't purchase in Capetown or Jo'burg if need be.
Your PH will supply you with a list of personal items recommended and will stipulate what they supply, it's not complcated, and it's all very civilized.
Most of these PHs are also travel advisors, shopping guides, babysitters, interpreters, bartenders, gun experts, story tellers, biologists, ornitholigists, drinking buddies, historians, diplomats (they never mention your misses and sing the praises of your good shots endlessly) etc............
It will be like no other hunt you have ever been on, Caramel, and make no mistake it won't be your last trip to Africa, it's highly addictive, equate it to "hunters heroin", something in the soil I think or maybe air.