Real World Involvement
I've read somewhere that the Leap Home Episode was written when Don Bellasario and Deborah Pratt were not getting along. Deborah was planning the Trilogy episodes and Don threw in this little piece to mess that up for her. Whether that is true or not, I have a real problem with the whole canon scenario and Donna.
1. I don't care for the concept "Donna will wait forever, no matter what." Regardless of whether Sam "fixed" things in Star-Crossed regarding her abandonment issues, that only addressed the situation from age 20 on. She STILL had the issue that she had dealt with (original history or changed) from age 10 to 20 when she truly felt abandoned. Thus, this would STILL psychologically affect her over the many years that Sam was gone. And if he NEVER came home (as stated at the last screen of MI), I can't believe that she would continue to wait forever. I'm as romantic as the next person, but I don't see martyrdom as romantic. Sacrifice, yes. Martyrdom is just pathetic.
And let's be realistic about the Beth thing too. In the orginal history, Beth has decided not to wait because she now believes (that is the key word) that Al is never coming home. When Sam tells her that Al is coming home, she believes this "angel" and waits. But think about it...if 50 years went by...would Beth still have believed him or would she have thought that Sam was a hallucination (that is would she have gone back to the belief that Al was gone and never coming home.) The only thing that Sam gave Beth was hope within a reasonable timeframe for her to hang on to. Al came home before she could lose that hope again.
In Donna's case, I don't believe she would believe that Sam is coming home forever. There would be a time that not knowing would get to her and she would be like Beth in the orginal history. He's not coming home, move on. Even if she still loves him (didn't Beth STILL love Al when she had him declared dead?) Otherwise, Donna and Sam is an absolutely tragic story in that Sam has caused the woman who was the love of his life to stay on a string...tied to him but never really having any happiness. Yeah...that sounds like real love to me (not!). Love would be Sam being willing to sacrifice that...to let Donna go because ultimately her happiness would be more important than his "keeping her for himself."
2. When Donna makes the statement in the Leap Back (Season 3) that nothing that Sam has ever done has hurt her, it was BEFORE some of the more troubling relationships (e.g. Tamlin and Abagail). Indeed, after Abigail, Sammy Jo comes on the scene and that had to be a issue for Donna. Might she have said what she said in year three so that "Sam would come home?" Yeah. Would she have NEVER changed her mind (especially after he doesn't return)? Doubtful based on the reality of the situation. I guess if Donna was some kind of a clingy, needy person without her own sense of herself...ok...maybe she "hangs on" to someone who never returns. But I don't get from either Star-Crossed or Leap Home that Donna is that type of person. Rather, I think that at some point...Donna stops being a martyr, and chooses to address her own needs.
3. REGARDLESS of whether Sam "remembers" or not...the whole concept of adultary needs to be looked. If I have amnesia and forget that I'm married to my husband and have a relationship with someone else...then because I am STILL married, I am committing adultry. The fact that I don't realize I'm doing it is not an excuse. Would it be understandable, perhaps...but I'd STILL be committing the act. Thus...as long as Donna is married to Sam, he is committing seriel adultry whether he realizes it or not. I find that entire concept a real problem in the storyline, especially when Sam is basically a pretty morally cool dude. The sacrifice on the other side would be Donna letting Sam go, so that he is NOT committing adultry. If he DOES return...could they get back together, sure. If he never does, then it is even more important that he had true freedom and not her "oh...I'm ok with it" which again is rather pathetic.
Anyways...that's my take on it all.