The 8th stage of the annual John Van Caneghem Tournament (2007).

Who can stop a player from replacing the Arcomage's ballista and claiming they completed the battle using it?

The record table shows "Cheater." Although I've never used artifacts and I'm not sure how closely the game monitors their use.

By the way, your description shows that you had several battles using the ballista. Approximately how many hours did those battles take, even with the ballista?

The ballista was only used in the battle against Malcolm because I managed to reduce his morale to zero. It took about 2 hours. Haart completed the battle in an hour using manual combat, but his morale wasn't zero.