With all that backup power in your bunker I would suggest a steel hatch and a good zombie riffle. They can sense the light and they love fish
Swedish-built is even better than Italian. I'm only half Swedish so you can imagine what the you get operating at 100%
Yes, 6500 Kelvin is the max for white light as that is the colour temperature of the sun. You add blue light to get higher Kelvin values. The R & G of RGB are red and green which have no value in reef aquaria. With LED you start off with the desired number of (usually 3 watt) 6500k white lights to get optimum PAR & intensity, then you mix in as many blue LEDs as it takes to get the colour temperature you want. Most corals require some blue light but for the most part it's cosmetic. Blue light cuts the yellow tint of aquarium water and makes it look clean & crisp.
You are correct, LED fixtures are very low profile so the duct would only interfere with servicing access to the tank. LED lights have no UV properties. MHL has long ans shortwave UV but most of it is filtered out by a quartz glass sleeve or a lens in the fixture.
In addition to the par values, you should find out the intensity (LUX) so you can make sure you are in the 5,000 to 10,000 range so you don't have photoinhabition (too much light, causing the coral to cease photosynthsis) and you can be sure that corals on the bottom are getting enough light for photsynthetic compensation (production of more oxygen & energy than what the coral needs for itself).