Re; Amy and Josh. I don't think they were any indication of Josh's feelings, or lack of, for Donna both times he was with Amy.
The relationship with Amy had any baring on J/D at all, nor should it have. I liked that they were able to have separate love lives while they were in limbo. It made them healthier, more realistic even. They are usually depicted as almost co-pendent in fandom but I think that is far from the truth. Not even professionally. I am re-watching reason 6, and I gotta tell you, Donna thrived professionally once she cut the cord. Josh floundered some, but it wasn't from lack of Donna, it was because he had a prissy candidate. Yet he got him nominated That is not to say they didn't miss each other. I just saw King Corn . But is besides the point.
Back to Josh and Amy, it can be argued Josh was ripe for the picking on the heels of Cliff Calley, but I don't remember there being any indication of this. I am not saying there wasn't any influence there, there was. But there was a lot of other factors that played into it. The White House was dealing with the MS reveal, Leo's depositions etc. I don't think the MS betrayal had sunk in for the staffers before they had to deal with the onslaught of the media and the cursed opposition. The reason why Josh seek Amy is the same way Donna did with Cliff, at that point, I imagine all of them needed something/something to take my minds off what they were dealing with for a moment.
Amy being a beautiful and seemingly smart woman have had something to do with it. Gotta give her some credit right? lol. I just think Amy could have been any other woman who made themselves available to Josh during that time. It is the same circumstances the second time. He needed an escape, she was there. Going back to what is familiar is easy and Josh is a glutton for punishment. She earnestly chased him if I remember correctly . Once they started having sex, why would he stop? He is a red blooded male, sex is sex. Plus the novelty was there till phone met stew. He'd forgotten that incident the second time. He was reminded rather quickly.
None of this had anything to do with Donna. Josh getting Amy'ed was a lot easier due to the J/D estrangement but being on a happier note wouldn't have stopped Josh/Amy from happening. I don't think. Amy provided a different sort of fulfillment that Donna couldn't. Same with Donna and the guys she was with. She is not so into Josh that she'd live a celibate life while she waits for them to happen.
Now had these things happened after J/D had crossed the line, I'd seriously question their so called love for one another. I think they were free to have healthy social lives before. They might have thought they'd always get together after leaving the White House. But without ever verbalizing these thoughts to a soul, let alone, to each other, they wouldn't know to wait it out. If they had, 8 years is still a long time to go abstain.
One of the beauties of them getting together at the end was that, the angst was played out. We'd never have to deal with them breaking up as a couple and dealing with them moving of however short lives. Blech.