B.S.
Lungs don’t “flutter”. Without the suction of an intact rib cage, the lung deflate to very small lumps. Not saying they didn’t try, but the results would be very disappointing.
Correct. Any "fluttering" (and it would be brief) could only be from blood still going from the heart into the pulmonary arteries and out into the lungs and then back to the heart by the pulmonary veins, by a still-pumping heart (which continues for a short period of time, at a greatly reduced heart rate, after a brain death or unconsciousness from shock or blood loss from other major wounds). But that assumes that the pulmonary blood vessels and heart and major blood vessels are not damaged in the opening of the back and pulling the lungs outward, and that not a great deal of blood is lost prior to that.
With tin-snips or other sharp cutting tools, the ribs could be quickly severed near the spine (the Vikings TV series showed the use of a hatchet or hand-axe for this, chopping the ribs), and then there would also have to be a second set of cuts of the ribs at each outside area of the back, to act as hinges to rotate the ribs outward (or to remove them totally). Otherwise, the springiness of ribs cut only by the spine means you couldn't easily expose or pull out the lungs. And there would have to be cuts of the muscles and connective tissues at the very top and bottom of the ribs "flap" (the rectangle of the back made by cutting the ribs at the spine and along the outer back closer to the arms) and also between the areas of the cut ribs themselves, in order to remove the "flap."
At best, there would be quick unconsciousness and death from shock and blood loss, and once the chest cavity is opened, there could not be regular breathing of the lungs, which need the closed vacuum-like space around the lungs in order for them to fill and unfill with air ("breathing"), so an asphyxial death would occur within seconds to minutes from that alone.