Burying is normal, and I've had some that have only come out at night.
Sea hares only live about one year, and you never know how old they are when you get them. I wouldn't feel too bad Sculpin if yours made it 6 months. Their life cycle is pretty crazy...so since they don't live long, they participate in mass reproduction (keeping the site a bit more family-friendly). LOL. Last I heard there was a university or some such that got them to successfully breed like mad down in FL, but they weren't available to the public yet. I'd be a big supporter of captive-bred sea hares, especially since we could roughly know their age (and not shell out $$ for a blue-dot one or something silly to have it die of old age a few weeks later.)