Which CAI are you using? If you are using mine what they are doing isn't a bad thing, they see you as a bigger threat if you are more powerful so they should focus you unless/until Isengard is the bigger threat. Sharing a border is probably the main cause of your relationship woes with Rohan though, in the main campaigns the Lore Accurate or GvE scripts keep reminding these factions that there even is lore to follow so this should overwhelm any negative factors. In free for all they just start with these lore relationships and nothing reminds them if I remember correctly. Rohan has no idea you have long-term good intentions in anything but the main campaigns, and that is all scripts doing that. Rohan has a stubborn CAI, they shouldn't give up the fight against anyone they start fighting if it is a threat, the Corsairs just need to be beat up a bit and they should cooperate with anyone for awhile is how their CAI is set to work. You can try giving Rohan the same CAI as Corsairs using a console cmd I don't remember and they may simmer down. The CAI's for the 'main' factions is diehard basically so they never cooperate with Orcs or whatever doesn't make sense in lotr. This is the reason for that if I remember right. Even in sandbox Gondor should come to hate the Orcs and stuff like that, Orcs are Orcs, just because Gondor never met them doesn't mean they will get along
. I already removed a whole buttload of M2TW scripts that cause positive/negative affects so not sure I can even do much more in terms of that without doing away with it all together for all factions.
Sounds like you want a script that adds .01 relation for all factions to offset the negatives, I use them in the First Age to create just such a thing, the ability to work with certain factions. I suppose it would fit in with Free For All as an optional script, I will add it to the list. Even in Free For All these factions, due to many factors like culture and crap, will inevitably fight. Even in the main campaigns Rohan and Gondor will duke it out when they don't have anyone else to fight late in the game. The CAI was designed to know this above all else, there can only be one winner in the end. The player knows it so I wanted to CAI to know it to and consider that in damn near everything it does. I did make a new optional CAI though for the First Age/Second Age that will be added to the other ages, the new CAI isn't near as selfish and sees the late game as a combined victory. This will not be the recommended CAI for the Third Age though since it isn't lore accurate to me anymore than GvE is.
More stuff will be added/removed to make all the custom campaigns different, right now the main focus for them is just keeping them working while I add in the First and Second Age crap to the mod.