Electronic – Beginner questions, 230v switch on PCB

Wow, Well we could spend hours discussing ground.
I'm not that much of an expert, but I have made lots of mistakes. First I would say there are two "ways" to think about ground. At low frequencies (DC to audio) the "star" ground concept is useful. Here I'll draw a picture, with big heavy lines for paths with a lot of current. And then bring all those separate paths to one point. (the point where power enters the circuit.)

The idea is to keep the high current's out of the more sensitive parts of the circuit.

At higher frequencies ~10 MHz and above, one starts thinking about a ground plane that is everywhere. With lots of ground connections between the case and the ground plane. At the higher frequencies on is not so much worried about the DC drop in the ground lines, but the inductance of the ground. You want the fast signal wires to have a (continuous) ground plane underneath them that carries the return current.

I'm sorry I haven't answered your question. Whether you have a loop or not depends on how you lay it out. And a small ground loop on a pcb may not be that bad.
The best thing is to build it and try it, and remember that you may have to "fix" the ground.

According to the data sheet, the 3 RGB LEDs are electrically separate. This means that you can connect them in series, using a higher voltage with fewer resistors and transistors. Nick Alexeyev's answer then applies. Assuming a 36 volt power supply, and strings of 8 for green and blue, 16 for red, and 24 for IR, total is 18 channels. I would not go with Nick's suggestion of 48v/12x strings for green and blue, since there isn't enough excess voltage for the limiting resistors to operate reliably, particularly with the Vf variations given in the data sheet. I'd expect that you'd need to measure the voltage drop of each string and tailor the limit resistor values accordingly.

What I think you've missed is power. Assuming 20mA for each LED, total power is 3.6 watts each for green and blue, 2.3 watts for red, and 1.5 watts for IR. Total power is 11 watts in the LEDs. I have no idea how you're going to heatsink this. Well, I do, but it involves using a beryllium oxide substrate for your LED PC board, bonded either to a pretty hefty heat sink, or maybe a TEC cooler. You want the LEDs to run as cool as possible for better lifetime. But trying to do it with FR4 is asking for early death of your LEDs. Similarly, you would also need to calculate the dissipation in your limiting resistors, although for the values I've given I'd expect total dissipation in the 4-5 watt range, and this can be handled with forced air cooling. And with the cooling requirements indicating a certain amount of increased size, I don't think you really need to worry about minimizing the driver board size, although at 18 channels you shouldn't have much difficulty.