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I would not be one bit concerned with those results. The battery drop you're worried about is an instant in time, when the starter motor has to overcome the inertia of an engine at varying rotational angles, and therefore, at differing stages of compression. The starter motor itself will come to rest at various rotational angles that will draw different amounts of current as it overcomes its own inertia, added to that of the engine. The spiky waveform at that point could be the brushes and commutators switching extremely low-resistance, high inductance windings inside, and perhaps cylinder compression loads.
The first revolution of the starter motor will draw very different instantaneous currents (with different waveforms) each time. A high-current scenario will pull a known low-capacity battery to lower a voltage, but only for a moment.
I don't see any evidence of intermittent shorts in there, except perhaps, worn and tired brushes and commutators. Nothing about that starter (in its present condition) will kill a new battery, and there is no danger of an old, worn battery damaging the starter. EXCEPT that one day, the battery may not have enough capacity left to complete a start.
Leaving your tired battery on the maintenance charger is a great way to extend its life. My 11-year-old battery was still cranking healthily with a resting voltage of only 10.5V. Keeping it on a GOOD maintenance charger extended its life by another 9 months, but eventually, it did leave me stranded.
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