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 …
The Smell of Molten Projects in the Morning 
Amusingly, I saw instructions on how to drain the oil in the manual of one of those (tip it on its side with the carburator up), even though it says you don’t need to. This is actually generally a faster way to drain oil than through the tiny drain plug even on mowers that have a drain plug (also oil drains faster when it’s warm).
I did that with the pressure washer (as per its manual), only to bloosh gasoline from the carburetor bowl all over the driveway. It is now defined to be “lubricated for life” …