Help identifying a voltage regulator

1: Yes, you can do that. Essentially, that's how power supplies work. They can handle multiple parallel networks within their current capacity. As for the heatsink, that depends on the regulator, the current draw, the ambient temperature, how efficient it is, etc. It's not a simple yes or no.

2: The capacitors depend on the regulator as well. Some require them all the time, some only require them depending on the input or output conditions, some never require them. The NTE1960 you linked to does not have an extensive datasheet, but is pretty similar to the LM7805. The capacitors are pretty much required for stable use. But these are linear regulators. Not efficient and they convert wasted energy into heat. Going from 12v to 5v, at say 700mA which is the high end for the RPI, that means 12 - 5 = 7v * 700mA = 4.9 Watts of energy being converted into heat. A heatsink would be required.

A Switching regulator is more efficient, in terms of both energy and heat. The OKI-78SR component you chose is a Switching Regulator. It shows that it would not need a heatsink in that same situation (Not in the engine compartment though, that's a different story). It is also a complete module, including the capacitors and the resistors it needs. It would be better.

3: A Car USB regulator would work just fine for your case, as long as the draw on it is under it's maximum. Some are 500mA, some are 1A, or better or in between, but some can't actually supply the amount of current it says it should, so you would need to test. The Model B has a 700mA draw/limit, the Model A is 500mA. Most of these usb regulators are switching supplies, and for your purposes, a car usb adaptor would be exactly like the OKI-78SR. At 4 bucks for the OKI-78SR (plus shipping) compared to a few bucks for a car USB adaptor, it really just depends on which you can get easier. Even retail, you can get a decent car one at any convenience or auto store for 10 bucks.

You could even gut the car USB adaptor for the board inside. Those things are so small now they are smaller than a car cigarette lighter, with the case, and the size of an SD card without the case.

Those are 12V fans, so you can power them using just the 12V from the PowerStation. No extra components needed. No need for a voltage regulator.

A voltage regulator will lower the voltage, what you don't need. It does this by converting the extra voltage into heat, what you don't want.

You only need to add components if you want to run the fans at a lower speed. E.g. to lower power consumption.

$$\dfrac{18Ah}{0.08A} = 75\;hours$$

However the current the fans will use also depends on the voltage provided. So they will use more when the battery is full, and less when it is "empty" (11V).

Just be sure to not let them run for that long. As the manual states:

Check the battery voltage regularly when using the 12V outlet. Do not allow the battery voltage to drop below 11V before recharging or damage to the battery can occur.

The PSX2 will NOT automatically turn off power to the DC outlet. Continuing to use 12V DC appliances after the battery has been depleted can permanently damage the unit.