but the burning gasses sets the whole thing in motion
Yes, by moving the bullet. Nothing moves until the bullet moves.
Now, I have been assuming from the beginning that we're not talking about catastrophic failures. Obviously if you shear the locking lugs then the system is no longer operating as JMB intended and all bets are off. I haven't really thought through the way things work when the gun actually blows up, and I'm not going to because it's a different mode of "operation".
Well, not really of operation--a different mode of non-operation.
Ok, I don't know what it is, but it's not the way things work when things are working.
Ok, that's all I'm going to say about stuff breaking--
The slide recoils because the gasses drive the case back against it while they drive the bullet forward.
The slide RECOILS because of conservation of momentum. That's what recoil is, conservation of momentum. And momentum is mass times velocity. Velocity implies motion and therefore nothing recoils without motion. Until the bullet moves there is no motion and therefore no velocity, and therefore no momentum and therefore no conservation of momentum and therefore no recoil.
ONCE the slide is unlocked from the barrel (which can only happen when recoil causes the barrel/slide unit to move far enough to unlock) THEN the slide may move backward under both recoil and also under the force of any remaining gas pressure in the barrel. My guess is that the bullet is pretty much gone by the time the barrel unlocks from the slide so that the remaining gas pressure is pretty much nil. So that gets us back to the slide moving only from recoil.
If you want to see if the slide moves from pressure or recoil, get a long thin punch and a pistol with a threaded barrel. Thread the barrel into something that you can clamp into your vice. Now put the punch down the muzzle against the breechface. Now whack it for all you're worth with a big sledge. You may shear the locking lugs or punch out the breechface, but you'll find out that pressure against the breechface won't unlock the slide from the barrel. This experiment might also convince you that the barrel and slide ARE actually locked together.
But you say, what about if you don't put the barrel in a vise? That's different--THEN, you're acting against the slide and barrel as a UNIT and they WILL move back together until the breech unlocks. But, pressure is INSIDE the chamber and can't affect the slide and barrel UNIT, only the breechface and the bullet. THAT'S the point. RECOIL affects the slide/barrel UNIT while pressure only pushes against the breechface from the inside without moving the barrel.
Think of someone standing in the chamber trying to push the gun open by pushing on the breech. He won't be able to because he's standing on the barrel which is locked to the breech. (Like trying to push your car while you're inside it.)
Now, if he can move the entire slide/barrel unit somehow then the breech will open on its own--but he could only do that from the outside--NOT the inside. Pressure (the guy in the chamber) can't move the entire unit. He CAN push the bullet because it's not fixed to the barrel or breech. By moving the bullet he can induce recoil. Recoil affects the entire unit and THAT'S what unlocks the breech.
Why do overpressured rounds swage the breechface irregularities or the extractor recess into the case head? Pressure...Why do pistols with excessive headspace blow the case heads? Because the case moved out of the chamber as the pressure drove it backward toward the slide which wasn't quite locked to the barrel yet...and the slide moved away from the barrel, creating a gap that let the case back out... and the area forward of the web lost chamber support.
Darn, you want me to talk about things breaking again. The forces exerted against the breechface and case head are MUCH, MUCH higher in this situation, and what you're seeing is the effect that OVERpressure has on metal. It can force the slide back by deforming the locking lugs enough to let the case head blow out, and also the OVERpressure against the breechface is sufficient to cause all kinds of deformation. Again, this type of thing is WELL outside the scope of the way the pistol was intended to operate and you can only draw limited conclusions from it.
Chamber pressure operates the locked-breech
Well, in a sense, chamber pressure operates any firearm, but that's only indirectly true in this case.
The blowback is "locked" by the spring and slide mass
It's not locked at all. That's why you have to keep the power low or make the slide very heavy. The velocity of the slide in a blowback is due to recoil (conservation of momentum) and if it moves fast enough, it will open before the pressure has fallen to safe levels. The proof is that blowback operation is not PRESSURE limited, it's MOMENTUM limited. You can make a safe blowback gun with a high pressure cartridge if you properly deal with the momentum by making a heavy slide. On the other hand, even a low pressure cartridge will cause problems if you don't properly deal with the momentum. And yes, the spring force does play into it. The velocity of the slide will be affected by the spring as well as the mass.