Of course it was a proxy war, and of course US bombing was brutal, not only to Cambodia but also to Vietnam.
> The US bombing of Cambodia was without a doubt a defining factor in this event.
Which event? US bombing was between 1970-1973. The genocide didn't start until after Khmer Rouge came to power in 1975, two years after US withdrawal as per Paris peace accords. On the other hand, China was the main sponsor of the regime throughout, not unlike their role to North Korea today.
They also actively involved with their "advisers" on the ground, from 1975 to 1979 when the genocide happened. So tell me what the defining factor was?
Talking about Sihanouk, he was hardly an innocent party so forgive me if I don't trust his effort to clear his conscience. He started cooperating with Khmer Rouge in 1973, giving them credibility given his enormous popularity. He became their head of state in 1975, albeit briefly. After Vietnam's army ousted Khmer Rouge, he again cooperated with Khmer Rouge at China's urging, knowing full well of their atrocity.
> You can blame China as well of course, but don't forget that the US government began aiding the Khmer Rouge at a later point in time again too.
As did all then-ASEAN's countries, all western countries, as well as China, when Khmer Rouge was no longer in power. They were against Vietnam's invasion, at the height of the cold war with the Soviet. Hypocrisy in the name of strategic goals.
Note that Vietnam wasn't an impartial party either. They invaded Cambodia with "volunteer soldiers", not because they cared about suffering Cambodia people but because they couldn't afford to have two enemies ready to wage wars at their border.