To summarize:
- You will have to clear immigration and customs at your first port of entry (in this case LAX). You cannot do this in SAN.
- You will be asked to clear immigration first, then collect your bags, clear customs and redeposit the bags for your connecting flt as you exit.
- At INS or customs there are no special lines for premium pax or FF. INS Pass if it works gets you faster but based on your posting I assume that INS pass may not work for you (it is good for frequent intnl traveller, especially for airports where the machines work).
- You do have a small edge as a business class customer only w.r.t. the economy pax of your flt. If there are other flts at the same time you lose that edge. (Being in AA terminal as opposed Tom Bradley helps some times and hurts other times).
- It will take you some time to get to the commuter terminal. FAs will not put you on the shuttle/bus to the commuter terminal. You can ask for help from AA agents or staff. There are plenty of them in the AA terminal. After you clear security (which is upstairs while immigration and customs is downstairs), you will go past Admirals Club on your right and past a bar and Chili's and go down using an escalator on the left side. An agent will check your boarding pass downstairs and direct you to the shuttle which is fairly frequent - every few minutes.

I fly through LAX on international routes quite often (AA is my preferred carrier and I am an AA EXP and lifetime Plat and SFO/SJC are preffered airports). When you arrive on AA flt in LAX, you arrive at AA terminal and not Tom Bradley. So you do not have to walk to another terminal. You come out of intnl arrivals of AA terminal, then go up by elevator or escalator to security and go to the gate for bus to American Eagle/commuter terminal. The flts to SAN are from the Eagle terminal, so you do have to catch a bus to get to the Eagle terminal. There are often (but not always) long queues for security, so using your AA status or business card boarding might help (they have a premier line for security while getting back into the terminal)

Being a US citizen or a premium pax does not get you any real edge in terms of the queues on your arrival (if your intention is to catch a flt with a short connection time). There are frequent connections to SAN, so if you miss your flt you will probably get the next one (be warned that often these flts are fully booked)