Yes I belive the ending is differant if you ACT evil throughout the game. No there is no way to go back and do the side quests once you have finished the game and if you want to solo I recommend using the charecter you finished the game with or the one man army, the monk.(Memnoch and the rudedawg know a lot more about soloing then me ask them)
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Sigmar has only one foe,
and he is Tiax!