And if the Chinese government did allow such an investigation, and it found China innocent? How many of those currently inclined to hate or blame China would believe it? Not getting into such a "heads we lose, tails they win" mess seems perfectly rational.
But in any case, yes, there are plenty of institutions in the West that would make such a cover up near impossible. The exception might be if the events happened within a military institution -- and even then, in the US or Europe, there would be calls for inquiries.
Of course if you're part of the "jet fuel can't melt steel beams" or "the CIA killed JFK" crowds, that won't convince you of anything. Reasonable people will however recognize that covering up such events under such scrutiny is a risky move, and as such unlikely to stand for long. The calls for more investigation would never stop, just consider how riled up antivaxxers are for no good reason; do you really think there would npt be a much larger number of citizens up in arms and furious if there was any hint of such a cover up in their country?
True, and I was obnoxiously snarky. But for something so high-profile and emotionally charged, the loss of face, loss of control, and other big downsides (for those in power) kick in as soon as a nation even pretends to allow a serious international investigation.
I will not argue that a cover-up in most Western countries would have nearly the prospects of success that one would in China (also not asserting that China is doing any meaningful cover-up on this). I will argue that those in power would (per Churchill) probably do the right thing only after exhausting every possible alternative. Note that most Western countries have had their citizens "up in arms and furious" for a good while now, over the endless screw-ups handling COVID. Sadly, that has neither ejected many politicians from office, nor caused a mass outbreak of government honesty or competence.
(Personal aside on the "jet fuel can't melt steel beams" crowd - both of my grandfathers were country blacksmiths. That profession is only possible because iron and steel get soft enough to work with simple hand tools at temperatures far, far below their melting points.)
[Edit - add the obvious missing carriage return after quoting parent.]