In my personal rulings, you can't use armor UD to negate damage from surprise attacks. The only way of avoiding damage would be the character rolling 3D6 DEX or WIS above a 25DL (damage = 25 - roll - stat). The complications could add more damage or receiving a narrative disadvantage, like a broken leg or losing your weapon etc
Yeah, you can use your background as advantage in your combat rolls, it depends on how you rationalize the fiction at play. To me, a veteran warrior has advantage against people with less experience, but not against other veterans.
And yes, rolling a complication in your attack triggers a UD weapon roll, if you are defending you roll the armor UD.
I'll add more detailed exemples in the final version of the book!