AFAIK there is no way to tell the ai to target specific cities.
Yeah, I don't think the AI cares for the objectives. -.-
I would rather have a memory that is fair but unfinished than one that goes on to a grievous end.
Rats. I thought I remembered reading about setting objectives for the AI a long time back. That's what I get for being out of the loop for so long. Here is another idea: if line of sight is a significant factor in AI conflict calculations, how about putting some towers on the Gondor side of Mordor, contrived so that they are on Mordor territory, and vice versa, so that each will have a line of sight to the other over the mountains. I don't think this is too great a problem lore-wise. It is reasonable that some towers would have been erected over the centuries that could easily have been restaffed at the consummation of hostilities.
I dont think that there is a work around for where specifically the AI will attack. Ai has some parameters in this game and they seem to be hard coded. So they will always attack conditioned to these parameters.
What you can do in terms of "force diplo" is what is suggested in this topic, delete the RK and adunabar agents so they cant make peace with each other... even so, they can attack you anyways, but they will be much more hesitant in doing so, because they will be already on another wars, specially RK (which is the most important lore-wise in the RK-Rohan relation).
Also, presumably there is this new feature of "perma ally" that is coming on Dom.