For reasons not relevant here, the lawn mower suffered some Foreign Object Damage:
I’m sure the hard stop loosened the tolerances along the shaft, but the mower fired right up (with that new blade!) and has no more vibration than usual, despite the seriously bent blade mount.
I no longer have a deep emotional attachment to lawn mowers, which is apparently common, as the label advises me there’s no need to change the oil:
Drive it ’til it drops …