If we go early in the morning before the gate office in manned, can we drive in to the Agate Beach parking lot

Yes you can. There's no gate so if there's no one in the little office you just keep driving

Came in late at night. Nobody to check me in. Set up and did it next day. Pretty casual place. Loved it. Was too windy to get out but wonderful gift shop museum and native village to see. Returning soon.

There is a checkpoint with a Ranger but it's not always open. You should be able to just drive in. I did this when I arrived late.

I don't remember having to go through a checkpoint to go to the agate Beach or are campground