There tends to be a lot of traffic around Carlsbad going south in the morning. If you aren't going to leave until later in the morning, I would encourage leaving at 9 or later. Leaving at 8 will get you stuck in traffic. The HOV lane is great, but ends before you get to San Diego County. There is a brief stretch of HOV lane around Del Mar, but it won't save you a lot of time because it's so short.
I know getting on the road early is difficult, but starting at 9, you won't get to SeaWorld until 10:30 at the very earliest, and probably closer to 11. Personally I would want to leave Anaheim by 7am.
FWIW, your drive back to Anaheim at night should be smooth sailing, as long as you leave SeaWorld after 7pm!