Why is my corned beef overly salty?
I think 2. is the most likely. Remember the beef has been brining for ages, a rinse will just remove the salt from the outside, and even then it won't do much.

Personally I wouldn't bother with the sous vide. 'Proper' corned beef should be gently simmered in a covered pot for about 2.5 hours. It doesn't need 'boiling' per se, and certainly not to oblivion!