I think that you should just wipe out the Scipii faction, they won't stop attacking you ( well at least on my Carthage camp. ) and peace i never managed to get, bot even a ceasfire.
however if you manage to take Messina (?) and defend it against the Scipii ( and even the Brutii ) you will be fin e.
takingn Capua won't even be a must any more, i don't know for shure but i think the ''anti-you'' missions will probably stop once you desroy the Scipii.

about the last thing you wrote ( the romans ending alliance and al ) I don't think that will happiong. in every campain I played they never broke the alliance they had, very frustrating...