Keep up the good work!
Yep - it can be confusing but the rule is that you never add your Brawn or Magic score to a dice roll unless you are rolling for 'damage score' (keyword being 'score'). The text will always make this clear.
For example, 'Cleave' states that you roll one damage 'die' (not score) and apply the result as health damage to all enemies, ignoring armour. So you wouldn't add anything to this as it is 'damage dice', not a 'damage score'. The text for the ability also states that you use this instead of rolling for a damage score.
'Deep Wound' however, allows you to roll one extra die for damage score, so in this instance you would roll two dice and add your Brawn as you are still rolling for damage 'score'. However, if you had both 'Cleave' and 'Deep Wound', you can't use them together because 'Cleave' replaces your roll for damage score whereas 'Deep Wound' adds to your damage score roll. (i.e. you wouldn't be able to combine the two to do 2 damage dice to each enemy.)
Lots of finicky rules to remember like that!