The rulebook says stress is wiped at the end of a conflict -- you're confusing conflict in the general sense with the conflict in the game's specific sense. According to the game's rules, a conflict is a battle. The first conflict in that scene was Ramirez and Dresden vs. Madrigal and Vitto. They kill Madrigal, and Vitto calls in the ghouls. That's the end of that conflict
No, it's a continuation of the same conflict with different weapons. Same battle, higher stakes.
There is no discontinuity in physical location - the entire encounter takes place in the cavern chamber.
There is no discontinuity in temporality - the entire encounter is continuous, without pauses, breaks, or interruptions, and the entities involved have to stay on their toes the entire time.
There is no discontinuity in the fundamental nature of the dangers and threats - I'd be willing to let a fistfight that turns into a car chase with gunfire count as two separate conflicts because the nature of the threats, and therefore the abstract 'stress', is so different between the two. But that's not what we have here.
And, of course, the conflict that was initiated was a duel to the death between two Wardens and two WCVs. The stakes are raised, but the basic conflict does not end until both vampires are dead - which does not occur until the caverns are blown.
If there were a single real example of a break, I'd let it suffice for a rest. Say, something along the lines of that scene from
The Phantom Menace where rotating force walls temporarily keep the combatants apart, and the bad guy paces while the good guy gets some meditation in. That would count. There was absolutely nothing like that. Everyone is constantly in danger until they escape the killing field.
Which is precisely why Harry does that kiss with Lara to generate the force bubble that saves them both. He was on his very last legs
at the very end of the battle and needed something, anything, that he could feed into the spell to power it. If he'd somehow cleared all his stress, mental and physical, he'd have been fresh as a daisy and wouldn't have needed to interact with a soul-sucking succubus to survive.