I was just at LLCA yesterday. It was 4 of us adults, and we were there specifically to shop. No one questioned us not having kids. I will echo what the others have said, and give you some more to go on.
The drive from Long Beach to Carlsbad will take you about 1 1/2 hours, unless there is major traffic. You should be against traffic most of the time, though.
It is $15 to park. As long as you buy $20 worth of merchandise, they will refund you the $15 parking fee. As long as you are in the park under an hour, they will refund you the entry fee. Time starts from the moment you enter the park.
The Big Store is under construction right now. They are expanding it's space. So they have moved the store to the Imagination Zone. It's a bit smaller than the usual store. They have tons of Series 10 and Movie Minifigures (but no Series 11). Other things I saw at the store, that stood out for me: Gandolf polybag, Z-95 Headhunter polybag, Town Hall modular.
There is a small kiosk at the front enterance. Mostly Friends and Star Wars stuff. There is a Movie store with most of their licensed merchandise. Star Wars, Chima, and other licensed stuff (no City, Space, Castle, etc).
The store I had the most hope for was the PAB store. It was disappointing. It was nothing special, with very basic bricks.
All in all, I was pretty disappointed with the visit. Maybe I'm used to times past, or maybe it was because I live/work within 40 minutes of the park. Also, you can't use your VIP points there.
I managed to get through all those stores within 40 minutes. I spent about 5 minutes looking at the X-Wing on display, and then headed out. Got back to customer service with 10 minutes to spare.
If you have nothing better to do, and it's worth your time, go for it.