18,000 miles on the first set and 12,000 on the second set while driving in a densely trafficked area seems low but not abnormal to me. Stop and go driving is hard on brakes. Lots of use and little time to cool off.
If I understand you correctly you have a bit less than a quarter of the remaining pad thickness difference between the one with the most wear and the one with the least. If this is the case I would say that is near the edge of "normal' wear.
If you are getting 12k+ miles out of front brake pads and the car doesn't pull or do anything weird under braking or have any tire issues I would probably accept it as "normal' for your car.
IMO You would certainly have to spend a lot more than the price of one set of brake pads to replace anything that might affect your pad wear and they would still probably wear the same way after you did it.
Also, it is not uncommon for brakes to get noisier as they get near the end of their life. Not hella noisier, just subtly more so than when new.
Scion, Toyota and most other Japanese manufacturers use spring steel tab "sensors" on the edges of their brake pads that contact the brake rotor when the pads wear to a certain level. Makes it sound like your car is eating its brakes every time you use them. The noise is designed to be obnoxious and get the driver into the shop to have the pads replaced before any metal to metal contact occurs with the rotor causing it to need replacement as well.
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