Once you begin a battle and lose, can you decide to use fate, and at that time decide to use your mercenary, or is the fate roll just a reroll of the original battle?

You have to use your Mercenary Follower to boost your Strength before any dice are rolled in the battle.

Exactly what I thought, but I wasn't playing when it happened so I didn't argue, but it's something I'm sure will come up again in different ways. You choose what bonuses you want to use before die rolls, then u may use fate to reroll if necessary, using the same stats. I think the only changes you might make would be to use a lucky charm or twist of fate,,,something that manipulates the die roll, but u can't change weapons, cast spells or use items to roll an extra die or anything else of that sort to change the base stats of the battle that is already in progress. U made ur choices on what to use for the battle and ur stuck with it.