I would use something like the RT8472 from Ricktek. It is a buck switching LED Driver which can take from 5V to 30V directly and will run up to 1A output current. It also costs $0.25/ea and has a dimming/switching input for easy control of the LED current output.
I would run the 6 LED's in series in one string. Failure of one LED is not so much a problem - LED's are diodes and they should not be designed for failure anymore than you would design any other part for failure like any other diode or capacitor or resistor. I always see people wanting to design around possible LED failure - IMO there is no reason for that other than either a bad design originally that is going to fail, or being burdened by the past when bulbs used to burn out (a bad way to do modern designs). After all, we don't design solid state transistors to be replaceable in modern electronics, even though we did with vacuum tubes... same thing.
I would heat sink everything REALLY well, then pot the whole thing with clear epoxy and forget about individual component replaceability in favor of reliability/waterproofing.