I think it's easy to explain why Sam doesn't remember Donna, for a couple of reasons. He didn't remember every key person on the project. In the very beginning, he kept getting Gooshie and Ziggy mixed up, but Al is always talking to Gooshie and telling Sam what Ziggy thinks Sam is there to do, so Sam remembers who they are. Likewise, I don't remember when Tina was first introduced, but Al talks about Tina all the time. The first time Al mentioned Verbeena Beeks, the project psychiatrist (or maybe psychologist, I don't remember), Sam thought Al was talking about some kind of drug! Sam doesn't remember Donna because Al never, ever mentions her.
Yes, he ends up remembering his parents and siblings, but they have always been a part of his life, so things that happen in his leaps trigger memories of them. And, in fact, he remembered Donna when he leapt into the professor in Star Crossed. But our Sam has no memory of being married to her because, in his life experience, he never was. It's a paradox. That's why they weren't supposed to tamper with their own lives. When he made his first leap, he wasn't married. He changed the course of his own life by interacting with Donna in Star Crossed and, when he went home in the Leap Back, somehow his altered life entered his consciousness, but he never lived that life. To me, that's the flaw. He shouldn't have known who Donna was. Well, I guess he might have recognized her, but he shouldn't have known she was his wife. He asked her how he could have ever left her, but she wasn't his wife when he left!
Whatever, it's a sticky situation, but just the fact that Al specifically never mentions Donna is, to me, reason enough to understand why Sam never remembers her. Remembering tidbits about your family is understandable, but most people would want to forget someone who left them at the altar!
I've already very specifically commented on why I think 'Star Crossed' is terrible and why it makes Sam look terrible, so I don't want to harp on that, lol. In general the Donna story arc is flawed writing and both episodes are flawed. Certainly the continuity is flawed. No doubt they weren't thinking at the time about TV shows being on DVD and that people would be able to watch them hundreds of times and analyze them. So they didn't worry about it.
While early in his leaping Sam forgot the key people and got them mixed up, post 'The Leap Back' he didn't anymore, so he should have had no problem with remembering Donna either at that point since she was there when he went back -- in my opinion. I agree, it's definitely a flaw in the writing that he doesn't remember both timelines, pre-'Star Crossed' and post-'Star Crossed' -- or like you mentioned he should remember just the first timeline if he's only remembering one of them. Maybe it is only because Al doesn't mention her - but that's a problem right there. The whole 'he couldn't do his job if he knew about me so you're not to tell him' doesn't work for me for so many reasons it would be another conversation. Also, since Sam still doesn't remember her or even know about her in 'Mirror Image' then it means he chose to never leap home without knowledge of a wife he's abandoned at home. He's therefore made a choice that was not completely informed, which is also a problem for me. And if it's because he's created a paradox by tampering with his own life then he needs to fix that -- because it wasn't only his life that he tampered with. He tampered with hers, too, and it's not like he had her permission.
(You know, there was a foreshadowing that he was married early on, in the Americanization of Machismo, at the very end when he's about to get married. He's concerned that he's not going to leap out before the "I do's" and he says "Al, I'm sweating here!" Al says something like "Not like you've never been here before." Sam gets a very concerned look and asks "Am I married?" Al quickly says "Just kidding!", but, as he turns from Sam, he rolls his eyes as if to say that he almost blew it. The conversation ends there as the bride enters and the Wedding March begins.)
I always thought Al was just pulling Sam's leg at the end of 'The Americanization of Machiko'. He is very careful throughout the series about what he does and does not reveal to Sam.