Activating Stalwart Defence takes a Full Action and a Fate Point. After that, it doesn't seem to require any actions to maintain, so you could use it while blazing away on Full Auto, correct?
The likely situation I see is the Storm Warden using this with a Bolter in one hand and a Chainsword in the other (paying 5 Req for whichever isn't standard). The character can parry every melee attack aimed at them and then either make an attack with the Bolter or the Chainsword each turn (except on the turn the Stalwart Defence is intitiated since that takes a Full Action), correct? This attack can even be a Full Auto attack from the Bolter, or a Swift/Lightning Attack from the Chainsword, right? If the attack is an All-Out Attack with the Chainsword, this would prevent the Parry options from the Stalwart Defence, but wouldn't end it, correct?