Welcome to the forums!
1. Deep wound gives you an extra die when determining your damage score. Any ability that refers to damage score means that you add your brawn to the attack. So in this instance, when you use Deep wound, you would roll 2 dice (the original die you would roll + one) and add your brawn. (You only add your brawn a single time, not to each die rolled - so if you rolled a  and a  and had a brawn of 5, then the final attack would be 4+3+5 = 12)
(If an ability refers to rolling and/or applying 'damage dice' then you do not add your brawn.)
2. Like all abilities, unless explicitly stated, rust only works once in the round it is used. So you can only apply it once (unless you had an ability that allowed you to restore and re-use the ability).
There is an advanced combat example on the downloads page, which you might find helpful in running through the use of abilities etc.
Hope this helps!