Cheapo, (& not a flame but an observation)
"generated those astronomical pressures NOT from in situ, but from a computer program which more or less extrapolated the pressure figures based on who knows what mathematical model(s)."
May be, but too, consider the effect of placing same load of a safe .357 loading into a .38 Speacial case. This is a pretty far "set-back" (.1" which is HUGE)) but does illustrate the disadvantages of reduced case capacity. Safe in .357 case will cause drastic pressure increases in a .38 spl case. Same with set-back to varying degrees depending upon case volume. 9mms are supposed to be very sensitive to even .030" depending upon load & bullet.
"But it also is often accompanied by loose neck tension (pressures go down)"
Loose neck tension in a semi-suto is what causes (allows) set-back when round is chambered. Feed ramp engagement & bullet gets set back (due to loose neck tension), causing lower volume in case = higher pressures. Progressive powders are exactly that. Lower volume = higher flame temps = higher pressure sooner.
"lots of room for gas and powder blowby past the bullet before it gets back forward to the throat"
I'd say that there is zero blow-by past the bullet as it seals the bore completely. If there's any "instantaneous" (subjective in time, of course) blow-by, it would be past the brass that hasn't a chance yet to expand fully and seal the chamber. Pressures don't build up (substantially) until the bullet engages the rifling causing the "back pressure" against the brass. The brass is the weak link in internal ballistics & is used specifically to seal the chamber against gas escaping backwards towards the shooter.
I doubt very much that any "swinging" of a cartridge would allow cartridge overall length to be increased. Even a very light crimp would hold a bullet in place for even vigorous swinging. A kinetic puller is certainly the way to go - for ~$20 it's a most welcome addition to (those who do screw up now & again as I most certainly do
&
) any reloading bench.
I usually check the taper crimp (emphasis added) for auto catridges by vigorously pushing with full force of hand against the loading bench. Measure THAT cartridge beforehand & again after pushing for crimp. Variance between bullet tips (in same lot - depending upon composition/construction) can cause very difference in measurements.
I'd consider even a .005" (measured across full bullet tip - to capture all tip inconsistancies) set back to cause me to add a bit extra to the crimp die.
& BTW, a mike (micrometer) isn't necessary. A quality dial (or vernier) caliper is sufficient to adequately determine any variance in "before & after" to .001". In fact, a (commonly encountered) micrometer has a rotating spindle which can interject measurement errors whereas a caliper is moreso straight-on & is more user friendly (when used properly - of course).