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First of all, I don't recommend you try any modifications like this unless you're experienced with electric circuits. So my advice is to forget it, unless you can get personal help with the design and installation.
Having said that, I can at least answer a couple of questions. This idea you have is very similar to the aftermarket angel eyes available for Bimmers. I know because I've installed some on my 328i. You can use a normal relay to trigger whatever you want, assuming what you're turning on is its own isolated circuit which has a fuse. I don't see the need for a optoisolator. Many LED strips are 12v with built in current limiting. You must calculate the total power consumption and install the appropriate fuse.
As for the headlights, your idea to measure the filament resistance is flawed. As soon as the lights turn on the impedance of red hot filament drastically increases. If you placed a resistor with the same impedance impedance as a cool filament in parallel with your headlights you'd blow the fuse immediately.
High frequency transients can be smoothed out with decoupling capacitors and you'll need to account for inductive kickback with a reverse bias rectifier diode.
If you want a 5w load on a 12v rail then you need a current of about 420mA (5W/12V). Using Ohms law to find the appropriate resistance (12V/420mA) yields about 29 Ohms. I am not confirming that a 29 Ohm 5W resistor is what you need to avoid frying the ECU or avoid any error codes, I'm simply answering your proposed electronics question. In reality you'd also have to account for the power consumption of the LED headlights and be aware of the rated current capacity of that 12V rail. There may certainly be other elements to consider. But typically to run aftermarket LED lights on headlights designed for resistive loads you need a parallel load resistor to satisfy the minimum load requirement.
Again, I don't recommend you attempt any electrical modifications unless you get help personally from someone with experience in wiring electrical circuitry. You don't want to go and fry your new car for a couple lights man. It's not worth the risk if you make a mistake.
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