We ended up going on Friday instead of on Wednesday. My brother rode FJ three times going on the single rider line.
We were in WWOHP around 2-5 and the wait times that day for FJ ranged from 30-90 minutes (but my brother walked right on with single rider). Wait time for Hippogryph ride was 15-20 minutes, and Dueling Dragons was 15 minutes. Waits to get into Ollivanders were about an hour, and to get into Dervish and Banges and Zonko's were about 15 minutes.
It was crowded but manageable. There were no waits to get into the area, TM said that this week and next week were going to be slow. Lines for the shops were too long so I bought my Hufflepuff scarf at the trading post store at the beginning of IOA.
And yes, you can take pictures (with flash) inside of the castle, just not on the ride.
As for the rest of the park, the lines were about 10-30 minutes. Single-rider lines are the best way to get a lot done in Universal I think, my brother rode Spiderman (35 min wait) 5 times in one day. Hulk had another 30 minute line, and the Seuss rides had 10-15 minute lines. Sometimes it would say 10 minutes and we would walk on.
Hope that helps!