Don't think it's even possible to win without smashing some bones, and it's extremely hard to make a merchant-like playthrough because experience is linked directly with killing enemies. Add to that the uselessness of troops in auto-calc, and as well on the battlefield. Your best bet would be to stack huscarls and rhodok sharpshooters as much as you can, and own enterprises literally everywhere, so having a single faction against you isn't that much of a deal.
About companions, their problem is that most of them hate each other if you aren't careful about your choice :
http://mountandblade.wikia.com/wiki/HeroesI'd go with Matheld, Nizar, Bashetur/Rolf, Artimenner, Lezalit, and Borcha. The problem is when you plan on pillaging villages (and caravans for Marnid), you get way less choice compared to what you cab get you could get on a "good guy" playthrough.
About the soldiers, get some trainer skill up, and recruit peasants from Rhodok and Northern lands. The problem with pillaging is that you won't be able to recruit anymore, so you better skip the villages which give these troops. You could also try doing some quests so they give you actual soldiers after some point.