The two situations were completely different: in 1943 Iran was largely occupied by the Allies (British and Russian) who thus had the final say in everything. Whereas in 1979 the Shah was toppled by a genuine revolution; at that stage there was nothing the US could have done for him. Perhaps if he had abdicated himself a few years before 1979 in favour of, say, his son, things could have worked out.
One must point out that the US did intervene in 1954 to topple Mossadegh, the Prime Minister - with the Shah connivance. However, that was also a different business.
As for the idea that the Ayatollah was an American creature—frankly, that's risible. But let's try to seriously engage with the idea and ask some questions about it:
What would have been the American motive? Since the Shah was an American creature of sorts himself, the US would need a serious motive to replace him with such an unstable and unknown quantity.
Are there any other examples of the US installing fundamentalist rulers anywhere? I don't think so.
What benefits have accrued to the US from the Ayatollah's regime? Nothing much, except a lot of grief and trouble.
In the more than 30 years since then, has any evidence of US collusion in the Ayatollah's rise come to light? Not that I know of.
So, this idea has to be dismissed out of court, upon examination.