Electronic – VUSB / +5V selector to safely power KNX project

Maybe a buck-boost? Like this for an example: http://www.linear.com/product/LTC3536

The LTC®3536 is an extended VIN range, fixed frequency, synchronous buck-boost DC/DC converter that operates from input voltages above, below or equal to the regulated output voltage

Using a lower voltage for the LED is mostly fine. You will waste less power in the resistor, but you trade this for less tolerance in matching the resistor and LED. The edge case is obviously where the resistor has to be 0 Ohms, and the forward voltage of the LED is 3.3V -- at this point, the U-I curve becomes fairly steep, so manufacturing tolerances will heavily affect light output.

Whether a linear regulator or a buck converter is more efficient for the processor and accelerometer depends mostly on the currents involved -- for small currents linear tends to be better.

You can usually mix 5V and 3.3V components with resistor dividers on the data lines (these are high-impedance, so the divider works), but you need a proper regulator for 3.3V power, and if you design that to be efficient, you are likely to be better off using a single voltage everywhere.

If all you need is full brightness, and cost isn't that much of an issue, the Allegro A4490 could work for you:

• using the voltage across the series resistor as the feedback voltage (\$\frac{800\mathrm{mV}}{20\mathrm{mA}} = 40\mathrm{\Omega}\$) gets you efficient current regulation
• turning the channels on with the high-impedance enable inputs reduces the current draw of the MCU, so you can use a linear regulator here (ideally, the MCU does not drive anything).

The downside here is BOM -- you need three resistors, inductors and flyback diodes as external components for that IC, and since your current demands are fairly small, it is most likely that there are better ICs out there.