I like this and it is consistent with this TL so I'll accept it as part of it. I like the bit about putting the two Vietnams together in a common cause against Lon Nol.
A couple of things: The US is also throwing support to Son Sann to diversify the Cambodian government. Even if that fails, its still likely to lead to some conflict.
President Ngô Quang Trưởng of South Vietnam is also a tough customer and not one to be easily cowed. He still has a military presence in Cambodia and would act to protect ethnic Vietnamese. His South Vietnamese Army is far more effective than Lon Nol's forces at this point.
Well, thank you. Oh, and the bit about North and South Vietnam protesting the riots and killings? OTL.
About Son Sann--to my mind, his big disadvantage is that, even with American support, he's on the outside looking in. He has support with a lot of the political class--but Lon Nol has the actual government, the army, and he's the one the people have heard of. (Glad to hear he didn't get caught in Beijing though.) As I figure it, Americans aren't happy with Lon Nol, but they don't want to risk losing what they've got. They'll support Sann, say, founding a party, and competing with Nol in elections, but not allow him to form an militia group, or bribe officers in FANK, or anything else that could destabilize things that much. The American government has convinced itself that Nol's unstable, and that his brother's a tad corrupt, but that they really aren't horribly evil fellows, largely because they really don't want to think what they might have done here. (And remember, this is one guy's ancedotal account. I've no doubt government officials will declare 'We had indications that things were bad, and we did what we could to keep things under control. And we SUCCEEDED--that crazy bastard only really got going when we left.')
As for President Trưởng--I've no doubt he's doing what he can in this situation. But he's only got so many troops there, he's got China and North Vietname to worry about and the Americans REALLY don't want the governments they're supporting to squabble too much. Plus, the Lons aren't having them gun down the Vietnamese in streets--they're keeping this small and low-profile at the moment. It's a village here, a shopkeeper there, and if the South Vietnamese or America catch wind of some of it, denials, excuses, and if they have to, a scapegoat. (Also, if the South Vietnamese REALLY start breathing down their necks on it, they back off for a while, and start killing the Cham, who have nobody particularly looking out for them.) Nol'll only go full-scale genocide if he thinks he can get away with it, or if he thinks he's going down.
As I tried to indicate, right now the Lons' Cambodia is slightly off. It'll only go full horror show once the American presence in SE Asia declines--and it will be a very different horror show than Khmer Rouge Cambodia.