Well...there are things that either side could have done to prevent it. The war could have been, at least, postponed by the selection of a different candidate in 1860 for president.
One of my best history professors said, simply, that sectionalism in the south - prior to 1860 - kept appearing every time that there was a controversy. There's a long run-up to the civil war - remember, all those compromises going back a half-century - and in each event there's the propensity for a large-scale war. But there were characters like Henry Clay that kept tying things together again, and eventually things cooled down.
The civil war would have likely been at least postponed if one of the moderate candidates had been elected in 1860. I don't think even the Deep South would have seceded if there wasn't a hope of the moderate states, like Virginia, for joining the Confederacy.
The hope for a player wanting to avoid the Civil War is to compromise long enough that slavery becomes economically unfeasible - or at least so apparently unfeasible that the southern states voluntarily give it up. And I think it could have happened, over time; all the other states of the world gave up slavery with conflagrations less than the ACW, and I think that given enough compromises the civil war could have been avoided.
This of course comes with the proviso that trying to wean the country off slavery would lead to a lot more dissent in the long run, and may very well hamper the player's ability to expand, and leave the state open to foreign designs, possibly. In short, not going for the civil war might leave the US in a bit of a bind, even though it never has to face a full-blown war. So, the player makes a choice.