You ask a lot of questions, each of which could have a whole "chapter" written to address them (as there are infinate variables, etc....).
Re.: the wet zone: I find that "pebbles" have nothing to do with metal target distribution. I've hunted lots of places where I see those domino sized rocks all assembled in a zone (that, perhaps, weren't there the day before), but find absolutely no targets there (or ...... molten aluminum can nuggets, at best). I know this seems to defy logic, because you would *think* that since those rocks are heavy (heavier than the surrounding individual sand grains), that they would be an indicator of mother-nature grouping coins there too, right? But it's not so. Because if you think of it, coins and similar metal items (rings, etc...) are QUITE a bit different than pebbles and coins. The metal item is dense and heavier (per size) and signicantly differently shaped, than a rounded polished stone, with lots of "surface area". So they will, in fact, follow different rules of placement in erosion. That's not to say they CAN'T be grouped with metal coins, but just that it's not necessarily a "given" that they will.
Your best bet to find the right wet sand to check, is 1) sand that is hard to the step, very firm, hard-packed, etc... Because that means sand has been "going out". If it is soft to the step, then it's "coming in". 2) Sand that is "lower" than the surrounding wet sand. This can get tricky though, as it's possible that the entire wet zone is "lower" (when compared to the day before, for instance). Are a single entire parallel is "lower". In each of those last two scenarios, it's harder to guage, because there is less to compare it too, in a side-by-side fashion. You just have to look at the slope angle, and know the "norm" for that beach. Once the slope deviates from the "norm", it can be a sign that sand is going out. Or when you stand on the wet sand, and look up and down the beach, you can often see the "indent" (scallop, or low spots, or wet spots, etc...) because you can tell the water came up further, and circulates through an area higher. Those low spots are the better zones on the wet beach. And they might have absolutely nothing to do with pebbles or rocks.
I wrote a whole 2 piece article on this, that you can find at the Kinzli California detecting forum. You're from CA, so you should probably just get on that forum anyhow, as there are other San Diego hunters who frequent that forum anyhow. To get past the guest page, you have to sign up. But it's easy: just pick a name and a password, and you're on. The article I wrote was from ~30 yrs. experience on the central CA beaches (which are closer to Alaskan swells/storms, and face NW and W, verses S and SW like yours. So CA (with your "Baywatch" style beaches) have a definate different "feel" to them. Thus some of the rules-of-thumb can vary, obvsiously, from beach to beach, state to state, etc... But some general principles can still apply to So. CA, I would think.
Yes, bay (Marinas, coves, etc...) will be utterly un-affected by the factors which affect beaches that face the open ocean.
The bulldozers you speak of, are only fooling around (for litter pickup, or berm building, or whatever) up in the dry sand. So they would have no bearing on the erosion-watch factors of the wet beach.
Yes, coins and targets do "come back in", after being dragged out by major winter storms. But my experience is, that to find targets on their way back in, is much more "random" and harder to find, predict, etc.... I'm sure there's a scientific principle to when and how it happens (d/t I've heard of guys who stumbled onto pockets of targets that were CLEARLY on sand that was building back in, and yet they knew they were older, d/t they contained old coins, etc...). More often than not though, spring beaches that are "building back up", seem to be sterile, or simply, random targets with no rhyme or reason. Contrast that to sand going OUT which can move massive amounts of sand, in a single night, creating an instant and clearly defined "riffle board/sluice box" effect.
For your San Diego beaches, which face S and SW, I would think you need to watch for the southern hemi swells (same ones the surfers look for) which bring the big surf and waves. When those combine with high swells, give it a try on the wet, looking for the landscape clues that tell you where to try.
