Combat Styles are specialized Combat Skills, and when using that Style, the skill bonus of that Style is your Attack Bonus, the same as the skill bonus for a more generalized Combat Skill (such as Bows) is your attack bonus when using that skill.
So, if Set was purchased for Archery (and not for Bows, since Combat Moves
MUST be purchased separately for each Combat Skill), then it can ONLY be used with Archery normally, not with Bows. And so the Attack Bonus will be based upon the skill bonus of Combat Skill it was purchased for (i.e. Archery)
And yes, Novus is specifically made to be flexible, so that there are usually options and choices for players to make in regards to acquiring/designing the character that they want. There is no "one way". Some ways might be more "min-maxed" while others are not, but the point is that it is the player's choice to make.