Exhausted Squads and Melee Combat

Good Evening / Morning!

I was hoping to get a little clarification on a few points:

1: Exhaustion - If a model is exhausted, can it still perform other actions? For example, if my squad is attacked in melee, and I declare I am not fighting back. It states that now since I have been attacked, my squad is exhausted (Page 18). May I still declare melee on my own turn with this squad?

2: Fighting Back and Actions - I was reading another form post about fighting back, which made me question how it actually worked. It was stated in a response to Saphyron that: “Fighting back is not defined as a reactionary action, but does use up 1 command value. So essentially as long as the squad has not used it's action yet this game round.... you may always fight back.”

Does this mean that a squad can potentially fight back multiple times in a turn? Or does declaring you are fighting back also use up the squads action (or at least make the squad exhausted), like I assumed it did.