Question: I'm trying to understand ways of isolating circuits and general concepts of isolation.

* I understand how high frequency signals can pass over a capacitor.
* I understand also how optocouplers work.
* I understand how transformers work.

Many places talk about using transformers to isolate,but as far as I know this is only for AC sources. They talk about using transformers for isolation but don't take into account how to isolate DC power sources.

How I can isolate a 48 V DC rail that feeds a DC-DC buck regulator with 12 V DC output? I can think of no way to transfer a large amount of DC energy without direct contact.

I understand induction, electromagnetic waves, capacitance, optocoupling, saw waves but I don't know why all this is related to galvanic isolation. Why galvanic? If for example I use 220 V AC isolation transformer, where is the galvanic current, or the galvanic "phenomena"?

Answer: 
In a word, no. The FFC cable would likely overheat and maybe melt or burn.

Both your connector and FFC cable are rated 500mA max.

Therefore if you have five separate 500mA constant current sources, you can't combine their return paths to a single wire, as it would mean 2.5A return current through single wire.

You have to have 5 return paths. Either separate return paths, or, if you combine the return paths on the LED board, several ground wires on the FFC is still needed, then a minimum of 5, preferably more, because if one or a few ground return wires make a bad connection, then all the return current would exceed the current carrying ability of the remaining wires.
That is fine to combine grounds and return the current through less conductors, but that could have consequences for the LEDs if they're being switched or dimmed.

The FFC cable has resistance and will create a small voltage, the more current you send through it the more the voltage.

For example: if you have an FFC cable resistance of 100 millohm and a current of 500 mA, you will have a 50 mV voltage rise on the ground. That is probably not going to be noticeable for most applications but if you went up to more of an amp it would be 100 mV and a sizeable portion of the LED voltage which would be noticeable on most LEDs.

If combining ground conductors you might experience cross-dimming if the LEDs are being turned on and off. Measure or calculate what the resistance is and how much current you are sending through the cable and see if it will be a problem.

Combining grounds is worse for common mode voltage also, and this could cause the LED PCB to radiate. If you are using non constant currents, or PWM combining grounds could be an issue from an increase in common mode noise