@Ingra
Well, answers in the same order:
1. I already said that I don't even want to see any 18+ material here - it would be just plain rude to the story, but there's
a lot more things you should do with your girlfriend, aside from sex. Actually, Zen already provided some pretty romantic moments for Shirayuki in the past (I'm not counting some scenes where he just wanted to be flashy, like with the "promise for a confession" scene long, long ago), but these are so rare nowadays...
2. I'm not talking about sleeping, but this is just not the way you should behave yourself when you're alone with your beloved one. Well, there's a perfect plot in a similar fantasy setting which provides many examples of how it should be. And you probably know what I'm talking about. Yeah, Spice and Wolf. The relationships between Horo and Lawrence are so much deeper and more mature that it's ever wrong to compare it with this at this point. And Horo can be REALLY naughty at times (given that she is a werewolf goddess having no shame at all), but even with this, their interactions with Lawrence could NEVER be called even indecent (I cannot even say 'sexual' here). They are an awesome sample of how you should care about each other.
The big problem with Zen and Shirayuki is that both of them have exactly zero experience when it comes to closely interacting with another sex. I wholeheartedly agree that they don't have time for this, period. But the biggest problem is that when they actually HAVE this little time together, they do nothing to fix it! I can bet 100$ that Obi will be furious in the next chapter when he'll hear about their little embarrassing chat together. Zen is already slow with his proposal (he's actually two years late for this, just remember their discussion in the harbor), but he's even more slow with the romantic moves. He was mad at Mitsuhide two chapters ago, but it's the pot calling the kettle black!
3. Izana already said it to Zen in an implicit way several times that it's okay for them to be together. He's even said that Zen is free to go to the Wilant (and remember what Shirayuki said to Izana after that ball). The interaction between Shirayuki and Haruto will be much, much more interesting, and I'm really curious about why it wasn't mentioned to Haruto when she was present at the coronation that her younger son have a favourite now. I'm also curious about the absence of Haki in the plot - she is really interesting character.
4. Well, it's the essence of the Izana's request he made to Shirayuki. I think he doesn't really care about the Olin Maris being accepted - he wanted an examination for Shirayuki about whether she is able to stand beside Zen as an equal or not. If yes, it will also serve as a perfect opportunity for making her face recognizable in the north, both as a scientist and as Zen's future wife. And in the case of success he will also have a new decorative plant in his homeland. Three birds with one stone. I LOVE how he handles things!
5. Zen is just being a kid with these matters, as always. Without Shirayuki out there to kick his ass in place he's never going to have word in love affairs. =_=