Possibly Talia's, too, in a weird way - canon seems committed to the idea that Ivanova can't be happy in any relationship ever.

Yep - because Susan was Sheridan's second, and no one else can compare. (grumble)

And I suppose that's what Lennier's betrayal scene was, or what it could have been with a few slight changes. If Delenn had thought any less of him after it, I'd definitely have been throwing things at the screen and shouting.

And JMS is just too good a writer - he had created characters (and a relationship) so strong that even he couldn't destroy them. Still, the dearth of D/L shippers in the fandom does seem to indicate that he suceeded to some extent.

That diary was such a strange thing that the first time I saw OaR, I was tempted to think Delenn was just making it up.

I thought the same thing. Even if there is something that could have been interpreted that way (like "Valen Sinclair says that the One who Will Be will marry a human" or, alternatively, "The diminishment of Minbari souls didn't start with Valen and therefore what Delenn is doing won't help" or even "Minbari religious law traditionally forbids marriage outside of the caste, and I would think marriage outside the species would be even worse") I just can't imagine Lennier saying flat out that Delenn made a mistake in marrying Sheridan. So even if she didn't make it up, I think she made it sound worse than it was. It may just have been the typical adulterer's instinct to save the marriage by putting the blame on the person they're having an affair with. Or a way to make Lennier seem pathetic, and therefore less of a threat (and not someone Sheridan needs to worry about having around.)

Or...if you see Delenn's behavior towards Lennier throughout the series as basically having an affair with him and asking him to take all the emotional and moral responsibility for it (which is certainly a plausible interpretation) then what she's doing here is just an extention of that. Maybe the diary actually says something like "I am starting to believe that Delenn loves me after all" and Delenn is so desperate not to acknowledge the truth of that, that she puts it all on him.

Although it would have been exceptionally OOC for Lennier to say anything against Sheridan, the way that he was close to Delenn in a way Sheridan just couldn't be meant Sheridan wasn't everything to her.

I think that's it. His existance is an implied criticism of Sheridan, and his relationship with Delenn points to places where Sheridan just can't or won't be there for Delenn. Which is one heck of a lot of Delenn's life, including all her past and all her life on Minbar.

Getting rid of Lennier wipes out part of Delenn's life, because there's no one around that she can share those parts of her life with now. It's terribly cruel to her, and I hate that we don't see how hard it is. Paradoxically, the hardest part might be how easy it is - that without Lennier, there's no one around who remembers her guilt, so she can pretend it doesn't exist. But that's cutting off a part of herself, and I will continue to maintain that she will not be whole until she comes back to it.