It's not a possibility, because he did not hold them captive. Stockholm Syndrome only refers to situations where people who are held against their will have positive feelings towards their captors.

Sure it's a possibility, but the burden of proof would be on you to demonstrate that it was Stockholm syndrome. You can't get by just claiming possible scenarios that explain something. You have to actually provide evidence that your claim is true (unless you're defending creationism)

It's also possible Anna Ardin is suffering from multiple personality disorder and one of her alternate personalities threw the party for Assange, tweeted about how cool he is, and invited him to stay with her 4 days after the rape.

We could make up all sorts of implausible, but theoretically possible scenarios.

But at this point, the most likely possibility seems to be that Ardin and W are jilted lovers who want revenge against Assange.