If this is true it seems odd though because the UK has a very generous, to the US at least, extradition treaty with the US so it would probably have been easier to ask the UK to extradite him. Why go to the bother of trumping up charges in Sweden?
if they are not trumped up why did they keep him in sweden for a month voluntarily let him go and are now saying oops we want to talk to him again
I don't know because, like the rest of, we're not privy to all the information. On face value it was deemed that crime he was accused of did not constitute rape and one month later this was reassessed and a rape charge was brought. It happens.
why did they pursue him with vigor normally reserved for murderers and terrorists
You seem to be suggesting that it's normal practice to just forget about people accused of crimes who leave the country. This isn;t the case and Sweden applied to the UK to return him under the auspices of the European Arrest Warrant framework that applies in any case where the potential sentence is one year or more which is clearly the case for someone accused of rape.
why is the uk threatening to attack an embassy over it
Presumably because someone at The Foreign Office, or whichever office issued the thread, is unbelievably naive or reckless.
anyone who thinks this is just about an unused condom is too stupid to live
You really can't contain the hyperbola sometimes can you.
why go with the farce with sweden rather than extradite him directly
well extradition treaties are not useful if you want to hold someone indefinitely without trial so your question is pointless
So without all the information you've completely discounted the possibility that he may actually have a case to answer it seems.
I'm not saying that he did or didn't commit the crimes but I'm also not saying that just because the whole episode suits the USA they must have engineered it. Perhaps they did but it's all speculation at this stage.