Sounds like your brakes are playing hard to get-firm one minute, ghosting the next. Classic intermittent haunt, and yeah, your knock-back theory smells right; single-piston floats are forgiving until they’re not.
1) Textbook? Close enough. Driveway zip-tie trick works for spotting runout wobble-clamp it tight to the knuckle, spin the wheel by hand, and eyeball any gap dance. For rotor vs. hub: mark both, flip the rotor 180°, and test drive. If the mush follows the rotor, it’s warped; if it sticks to the wheel position, hub’s the villain.
2) Trapped ABS air would be more consistently squishy, not just post-jolt. Your double-bleed with scanner rules that out unless the HCU’s got a secret compartment.
3) Collapsed hose? Nah, that’d drag or pull like a bad date, not this selective sink. Hoses usually weep externally before going full balloon.
4) Low-drag calipers amplify knock-back with even minor runout-aim for under 0.002″ total indicated runout (TIR) to keep it chill. Anything over 0.005″ and you’re chasing ghosts.
5) Hub first: zip-tie pointer on a straightedge against the hub flange, spin and measure variance with feeler gauges. Then bolt on rotor and repeat-difference is your runout. Shims are a cheap band-aid if hub’s off by a thou; don’t hub-swap unless it’s >0.003″ or gritty.
6) Axle nuts love to loosen on unit bearings-re-torque to spec (usually 150-200 ft-lbs, check your FSM) before blaming the bearing. It’s 10 minutes and often the free fix.
Bonus: Rear drums out of whack would mush consistently or lock up in reverse, not this bump-triggered nonsense. Adjust ’em anyway; it’s like flossing-easy and prevents bigger drama.
Not saying skip the bearings, but chase runout first. Saved me from a $500 wild goose chase once. What’s the mileage on those hubs?