I think it is impossible to say what information would have 'came out' in any trial of Khmer Rouge leaders at an earlier point of time. As someone who has paid a great deal of attention to the ECCC I could posit that any information not pertaining to the crimes that the accused were on trial for, would not likely come out in court.

US financial support for the Khmer Rouge on the border with Thailand after 1979 is well documented but perhaps more limited than a few of the statements littered throughout your question suggest. This answer goes into some detail about UK/US support for KR in this period if you would like to look into that, but your question more relates to trials and things of that matter.

You state that the reason the trials were delayed so long is because of the fear of this dirty laundry being aired, while I think that view probably holds some water, it neglects the more mundane aspects of international relations, the intricacies of bureaucratic dealings between a real grab bag of actors in this instance.

But to just, slightly take to task some of your statements here... Pol Pot and the Khmer Rouge, for the most part, were not put on trial in any immediate aftermath of their regime, because they were not able to be captured. The civil war between their forces and the Vietnamese was ongoing, and as you said, the KR were able to stay in this fight due in large part to the sino-soviet split and the realpolitik involved with China / US rapprochement. But that funding was for this civil war, the US had not supported the Khmer Rouge during their time in power, if a trial were to be held prosecuting the leaders of the movement, the scope of the trial (like it was for the ECCC) was the period in which the Khmer Rouge controlled Cambodia.

The trial to bring them to justice could not have begun in 1990, at this time the KR were still hoping to usurp the former PRK government and insert themselves into leadership once again. The international community focused on bringing a kind of peace to Cambodia before bringing justice.

Following the 1994 UNTAC attempts to negotiate a democratic transition of power, the Hun Sen regime equivocated between wanting to have a trial and not. Meanwhile the US, for their part, by 1989 had begun openly discussing the matter. Here is a quote from Craig Etcheson's fascinating book on the trials:

Yet, in response to pressure from the U.S. Congress, the Bush administration had conceded that it appeared acts of genocide had taken place under the Khmer Rouge. “The administration has been exploring a variety of means for bringing Pol Pot and other discredited leaders to justice,” the State Department legal advisor wrote to Congress in 1989.

“From a legal point of view,” he told Congress, we continue to believe that the trial of Pol Pot and others by a future government of Cambodia, formed after elections, would be the most effective procedure for imposing punishment. Pol Pot and others have committed numerous acts that we assume are crimes under domestic Cambodian law. Moreover, Cambodia has been a party to the Genocide Convention since 1950 without reservation. Because Cambodia is the place the acts were committed, Cambodia is the only country with an obligation to bring domestic prosecution under the Genocide Convention, as provided in Article VI.

As to the nature of the crimes in question, for the first time, the U.S. government officially described them as genocide. The legal advisor noted that the UN Special Rapporteur on Genocide had reported on July 2, 1985, “that Pol Pot’s Khmer Rouge government of Democratic Kampuchea was guilty of genocide ‘even under the most restricted definition.’ … We agree with that assessment.”

Now, throughout the 90's Cambodia was quite volatile in relation to this transition from the PRK into UNTAC and then into the current Hun Sen regime. It would not always be the case that pursuit of a trial, international or domestic, would be beneficial to that process. High ranking members of the Khmer Rouge, like Ieng Sary, would be pardoned by the Hun Sen government in an effort to try and split the remaining Khmer Rouge. Often, it would negate the need for a trial from a Cambodian standpoint if it meant the possibility of the dissolution of the Khmer Rouge was prevented.

For the UN, the results of UNTAC had been mixed to say the least, and they needed a degree of stability in Cambodia politically before they would commit to a trial. This was, and still is, a difficult endeavour, particularly when they were negotiating this process with the Cambodian regime:

Exactly one month after the UN Group of Experts departed to begin drafting their recommendations, the last two remaining senior Khmer Rouge political leaders, Nuon Chea and Khieu Samphan, surrendered to Hun Sen. This signal event marked the final collapse of the Khmer Rouge political organization, and it occurred almost twenty years to the day after the launch of the Vietnamese invasion that had driven the

Khmer Rouge from power. Hun Sen had already anticipated the conclusions of the UN Group of Experts, and told reporters on December 3 that an international tribunal for the Khmer Rouge was “impossible.”

Moreover, with the final collapse of the Khmer Rouge as a political entity, Hun Sen began to contemplate the possibility that any tribunal at all was unnecessary. He had achieved his objective—the political death of the Khmer Rouge—and so the tribunal could now be seen as an idea whose time had passed.

Hun Sen would go on to famously say that Cambodia needed to 'bury the past':

If we bring them to trial, it will not benefit the nation, it will only mean a return to civil war.… We should dig a hole and bury the past and look toward the future. This is a pure Cambodian solution. We know the right medicine to use to cure

this problem. If any foreigners try to add something, it will never go away. If the wound no longer hurts, we shouldn’t poke a stick in it and make it bleed. Should we kill Khieu Samphan or welcome him for national reconciliation? This deal is a bouquet of flowers for this pair, not a bullet or a pair of handcuffs.

I feel this answer may get too far into the weeds, but I guess the thing that I want to get at is that I see this tendency, very often, to remove the principal parties of the Khmer Rouge regime and their terrible actions in power, from those that actually planned, participated in and carried out these terrible actions: Cambodians themselves. Yes, other powers have their part to play, the Cold War was felt in Cambodia like almost nowhere else, but that does not mean that they were not responsible for their own actions, and this can be extended to the idea of transitional justice as well.

The Khmer Rouge was not an invention of the United States, nor was it an invention of the Chinese (although they certainly had a larger part to play in supporting the regime ideologically, militarily and financially than anyone else). It was a Cambodian invention, it was a Cambodian regime. Likewise, the process of 'justice', involved them just as much as it did international actors whom they called upon to help at various stages and rebuffed at others. The reason that the trial is now utterly at a standstill and took many more years than it should have to conclude, is less to do with Chinese and US fears of 'bad things coming out' and more to do with the interference from the Cambodian government, who may be principally concerned with their own 'dirty laundry', rather than that of the world's superpowers, being put on display.