My understanding of mean high tide in California is the law does not specify a time frame for the mean. So how do you actually know where the MHTL is?

Generally there's debris left from the high tide - seaweed and what not. It's usually pretty easy to tell.

Yes but that would be the mean high tide of perhaps the last week. If a stormed rolled in, by your logic, the whole beach could be public property. Some states define the mean high tide as the mean over a 18.6-year period or Tidal Epoch. To my knowledge, California makes no such distinction.