I'm in the process of trying to figure this out. I've triggered alot of event in this game, but don't remember if this is one of them. I created a character and assigned him to Liu Bei's force and made him Guan Yu's son and started in 200. the first event if you are working in the city just doing domestic duties is Cao Cao coming to attack Xia Pi. after you've allied yourself with Yuan Shao. Then you will move to Xiao Pei.( city spelling might be off a little bit,sorry) after about a year game time Cao Cao will attack again and your army will be taken over with no control from this happening. everyone will move to Ru Nan except Guan Yu and Liu Bei. Guan Yu ends up with Cao Cao and Liu Bei is with Yuan Shao. This is when the battle of Guan Du should take place. I went to Ye to talk to Liu Bei and we ended up in Ru Nan after the event triggered but there was no battle. The only way I think you'll be able to be invovled in the battle is to be either Guan Yu or Liu Bei themselves. I hope this helps. Will try and get you more info after this senario is beaten. I'll try to trigger it again with one of the two next time. If you have anymore problems trying to trigger events please let me know. I love the challenges. ROTK X is my favorite game and I think I've come across just about everything there is in the game.
I get all those events you mentioned, If I remember correctly (With the one time that I triggered it) Guan Du should happen after the events of the Guan Yu-Liu Bei meeting each other in the battlefield. In other words, Guan Du should happen when Liu Bei and company are already in Ru Nan, however, 9 times out of 10, a Campaign will be declared by Yuan Shao on the Central Plains (Because Shao gets that city next to Luo Yong by event if it's not occupied, from the event of Guan Yu slaying Yan Liang and Wei Chou, forgive the misspelling if I miss pronounced their names) Obviously when the Campaign happens it just screws up the event chain because a lot of random things can just happen in that which is annoying.
I could have sworn I triggered the event once, it was more like the battle events that you see with the armies marching, etc..
However, the more I think about it, the more of a possibility is that I probably triggered it in VIII and not X, as Game FAQs does have some Guan Du events for VIII, but I still honestly could have sworn it was X!!!! , lol.
Ohh, and I have tried playing as Liu Bei, and Cao Cao, but I haven't tried Guan Yu or Yuan Shao, and if I recall the one time I triggered it may have been with Guan Yu actually, but again it may been VIII though, but still a no go yet for X, lol...
Well, I guess there's always Dynasty Tactics 2 for Guan Du (I'm going for a litlte change a pace at the moment, hehe)
but still if you happen to get it, omg please tell!, thanks!