Hm. That Brit WIA number, as you noted, is weird. Do you think it would be a better idea to keep it as-is and assign some kind of effect to "having more KIA+WIA than your entire army", like "total postbattle disintegration of the unit" or something? Or insert a threshold for the casualty numbers, double-checking them against the original?
The error isn't too terrifically removed from Wiki to be worried about - certainly nothing like the Great Siege of Gibraltar error. I agree with most of the assumptions - neutralizing CEV, for instance. But I'm curious about what effect flatter terrain than "rolling" bare would have. The area around Isandhlwana is dominated by one really weird looking rock (on which they didn't fight), and the rest is more or less flat. At the same time, most of the paintings I've seen of Isandhlwana make note of a medium-heavy fog on the battlefield. Have you run it already with those two things changed up? If not, could you try that?