A few things.
Donner probably would have waited until he was within charging distance of anything before he rushed out. He wouldn't have partial charged out into the open if nothing was within fifty feet and been unable to get to anything. I was thinking full charge 100ft. on the first full-round of combat. But we're ahead of that now, oh well.
Being mounted, would my mount make the check as being overrun rather than me? I.e. medium, +4 four legs
In a military saddle, would I have a chance of staying in the saddle if knocked prone (as per overrun)? Ride check (stay in saddle, DC 5)?
How far from my mount am I actually?
He'd probably would have withdrawn to his mount if anything and mounted depending on how far from grizzle I am.
Also, he wouldn't have used his Kukri unless he somehow lost his lance, his flail and his longsword or unless he knew that it needed silver to be damaged effectively.