Sure, the Khmer Rouge was preferable to Sihanouk. Just like Castro is better than Batista. Just like Mao was better than Chiang. Just like Lenin was better than the Czar.
If i understand you correctly, your premise is that if we support the King, he'll stay anticommunist and somehow morph into someone supportive of a democratic government.
I just don't see that happening, and moreover, if the UN can be made to disarm the Maoists (the idea of the UN making anybody do anything is a bit of a giggle, I'll grant you) and if they enter the political process, they will be swamped by the other parties.