Could the possible 3rd person who might have been there been the one who knocked on the garage door at 1am? Maybe in panic over what happened...possibly an accident occurred with the 3rd person and it wasn't foul play. Maybe they were not even at the van and the accident happened down the beach? The house was a half mile east of the van if you go the beach route vs road.
Heck, SS and the 3rd party could have left Az in the van and walked for a while and discovered the accident. Or Az could have been walking with them and they weren't paying attn to him as the tide was rising...he slipped.
There is a cove there and perhaps they were climbing over it, he slipped on the rocks or when they were walking back the tide had risen in the cove and Az fell in by accident by the time they were heading back or something similar.
Maybe they put the trunk up to sit there...went for a walk, etc...
Photo 1 shows the cove and past the cove..perhaps they walked and came back and tried to cross the open area just pass the cove and the tide had risen....trying to cross that Az got caugh up in the tide.
All this would account for why she did not run up the road for help or honk the horn over and over.
High tide and low tide cove reference photos:
Red lines indicate where they drove in at the end of the small road and then where the van ended up...