Joseph, I think the externals of the Christmas and Easter stories (virgin birth arranged by God; agony, death, resurrection, again arranged by God) are pretty much equally coherent. (Coherence isn't their problem.) But the point of each story, for Christians, is something much harder to swallow: Christmas is supposed to be about the Incarnation (with Jesus somehow being entirely human, just as much as we are, and entirely God, etc.) and Easter about the Atonement (where the whole death-and-resurrection thing somehow enables God to forgive the sins of humanity when he couldn't before). Both seem pretty incoherent to me.
They both make good stories, if you don't think too hard about how they're supposed to work. I'm not sure that has much to do with their coherence. (Take a look at the "explanation" in TLTWATW for the Easter-like event. Lewis isn't even trying to deal with the really doubtfully-coherent bits, but he still resorts to entirely arbitrary stuff about Deep Magic and Deeper Magic.)
It's quite well established that stories tend to feel more plausible if they include a wealth of details, even though the presence of those details actually makes the story less probable. (It's more likely that you'll be abducted by aliens than that you'll be abducted by aliens so that they can perform weird sexual experiments on you.) So I'd be very hesitant about taking the fact that a story can be told satisfyingly as a sign that it's less improbable, or more coherent, than a story that can't be told so satisfyingly.
People who grow up believing certain things,
even if they later stop believing them,
may not quite realize how the beliefs sound to outsiders...
(SCENE: A small cottage in Nazareth.)
Joseph: Mary, my dearest fiancée, there's something I've been meaning to talk to you about.
(Mary's shoulders slump. Slowly, as if under a heavy burden, she turns around to face Joseph.)
Joseph: You seem to be getting fat around the waistline, and throwing up in the morning, and, er, not getting any periods. Which is odd, because it's sort of like -
Mary: Yes! I'm pregnant! All right? I'm PREGNANT!
Joseph: How is that possible?
(Mary's shoulders slump further.) Mary: How do you think?
Joseph: I don't know, that's why I'm asking you. I mean, you're still a virgin, right?
(Mary looks up cautiously, and sees Joseph's face looking blankly puzzled.)
Joseph: Well?
Mary: God did it.
Joseph: You had sex with -
Mary: No! Haha. Of course not. I mean, God just snapped his fingers and did one of those miracle things and made me pregnant.
Joseph: God made you pregnant.
Mary: (Starts to sweat.) Yes.
Joseph: Mary, that is just so... completely...
(Mary's eyes squeeze shut.)
Joseph: ...COOL!
(Mary opens her eyes again, cautiously.)
Mary: You think so?
Joseph: Of course! Who wouldn't think so? Come on, we've got to tell everyone the news!
Mary: Maybe we should keep this between just the two of us -
Joseph: No, no, silly girl, this is way too important! Come on!
(Joseph grabs Mary's wrist and drags her out of the house. SCENE: The gathering square of Nazareth. A dozen well-dressed men, and the town's head rabbi, look on Joseph and Mary impatiently.)
Rabbi: What's this all about, Joseph? I trust there's a good reason for the fuss?
Joseph: Go ahead, Mary! Tell them what you told me.
Mary: Um... (She swallows.) God made me pregnant.
Rabbi, looking stern, yet understanding: Now, Joseph, you know you're not supposed to do that before -
Joseph: No, no, you don't get it! She's still a virgin! God made her pregnant directly!
(There's a long pause.)
Man #1: So, what you're saying here, basically, is that Mary tells you she's a virgin.
Joseph: Uh huh!
Man #2: And you haven't had sex with her.
Joseph: Uh huh!
Man #3: And now she's pregnant.
Joseph: Precisely!
Man #4: So you think that God did it.
Joseph: What other explanation could there be?
Rabbi: Joseph, that is just so... unbelievably...
(Mary holds her breath.)
Rabbi: NEAT!
(Mary exhales.)
Man #5: A miracle! A miracle right here in Nazareth!
Man #6: Wow! I thought that miracles only happened in Jerusalem!
Man #7: Come on! Let's spread the good news!
(They depart. SCENE: Mary is alone with her friend, Betty, in Betty's house.)
Betty: "God did it."
Mary: I panicked! It was all I could think of!
Betty: So who's the real -
(Mary lifts an eyebrow significantly. There's a brief pause.)
Betty: Ah. So that's why the rabbi went along with it.
Mary: Well, he thinks he's the father, anyway. Why, does it matter?
Betty: It puts some things in a different light.
Mary: Like what?
Betty: The rabbi has been telling all the pretty young girls that you, Mary, are the ultimate embodiment of feminine virtue, and when they grow up, they should be just like you -
Mary: I just feel so awful about the whole mess. What kind of thing is this to have hanging over my child's life?
Betty: You've got to put things in perspective, dearie. You told one little white lie. It's not as if you caused the fall of the Roman Empire.
Mary: But what if the Romans hear about it? I don't want my baby to end up being crucified!
Betty: No one's going to obsess about it that long. In a couple of months this whole thing will blow over.
Mary: I hope you're right...
(Exeunt Omnes.)