I got the replacement sump from AppliancepartsPros.com, which was priced at least $35 cheaper than anyone else. I watched the video and went to work. The video was super helpful and got me through it - it was detailed and accurate. It turns out my model was a little bit more difficult to actually seat the sump in and secure with the bottom latches, but after some sweat and cursing, I got it done.
The leak did, indeed, appear to be from a degraded seal/ring that surrounds the diverter motor shaft. Replacing the sump was the right move.
I was super pleased to save hundreds of dollars on calling a repair person or even more if I replaced the dishwasher.