sweet deal Andrew! I'm happy for you that the car is getting to the finish line!
When I r&r'ed my old shocks for the new ones I got the same reaction from the ride control light. It took a number of times clearing the codes in my case to get the system to calm down. I do not know why. I knew that I had the mechanical steps done correctly and the electrical connections were good. Yet for the first 20 miles or so I got every DTC in the chart!
I just kept reading them & clearing them and it stopped happening eventually. I still do not know why or if it's just that I disturbed the system. It was just strange & drove me nuts for a day or two.
On the pinion leak. Did you notice on the pinion shaft where the seal rides any pitting or rust or even a 'groove' from wear? Sometimes a crocus cloth and some WD40 can clean it up enough so the new seal has a chance to seat. If the pinion shaft is in good condition, the vent could be causing an issue if it is clogged....but unless the old fluid was like tar I doubt that vent would be at fault.
Tom