if she protects on an opponents turn does she ready herself, so basically she readies whenever she protects and when anyone attacks?

she doesn't ready when she protects. She exhausts when she protects. But every time an opposing ally attacks, she will ready herself and can protect again.

does she ready herself in time to protect against that attack, or is it too late?

She will ready when the attacker window opens. You have the chance to protect when that window closes.

so essentially she can protect for each attack assuming there isn't something on the field preventing her from readying. For more info check out the comp rules section 602

I have a question related to Donna. There is a priority point after declaration of attack and the attack becoming legal, correct?

I ask because I want to know if this is a legal play:

My opponent complete's two copies of For The Horde and then proposes an attack with Moko into Donna, before the attack is legal I play Face Smash on Donna and exhaust to destroy his Manslayer, then the Attack Window opens and Donna readies, then when I have priority again I exhaust to destroy Moko.

Basically what I'm saying is: Is there a legality check like in VS?

There is a combat proposal step before the attackers exhaust. So what you are suggesting sounds very possible indeed.

Not (yet) a lvl 1 judge or anything, but here's how I see it:

Quote from the v1.02 rulebook:
601.1 A player may propose a combat only during his or her non-combat action phase while he or she
has priority and the chain is empty. Proposing a combat follows the applicable rules for adding a
link (707) and adds a proposal effect to the chain.

This puts the combat proposal on the chain. You then link it by playing Face Smash and exhausting Donna, destroying Manslayer. Then comes:

602.1 As a combat step starts, the proposed attacker exhausts, becomes an attacker, and starts attacking.
The turn player becomes the attacking player. None of this uses the chain. Then, a priority
window opens, called the attack window. Any effects that triggered when the attacker attacked
are added to the chain, and then the turn player gets priority.

Because of the bolded section, Donna's effect would trigger, causing her to "un-tap". After this, you could, in response to the attack, exhaust Donna again to destroy his Moko. Yes that is a legal (and smart) move.

Hope this helps!

Donna Calister:
Protector
When an opposing hero or ally attacks, ready Donna Calister.

Face Smash:
Target ally has "Exhaust-->Destroy target ally" this turn.

wow that's awesome

yup it's fun being able to read the rules and figure out some crazy but cool combos to do (like that one)