The uneven slip-ring wear (one groove deeper than the other) is a common issue. I've seen it on every old alternator I've seen.

Fundamentally, it is due to the positive ions on the positively charged slip ring surface being attracted away, toward the negatively charged brush. Once there, on the one hand, they wear the slip ring more and on the other hand, they left the slip ring, making it smaller.

Conversely, on the negatively charged slip ring, any positive ions that evaporate from the slip ring surface will just hang around where they should be. Slowing the rate of evaporation and not accelerating wear.

Swapping polarity will be a great wear extender if it can be done easily and reliably. Slip ring replacement can be difficult, mainly due to the fragile yet critical field wires that you need to get access to and then connect to.

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2001 X5 3.0i, 203k miles, AT, owned since 2014