Sounds like you've already read the whole book. I'm only basing my judgment on the first four chapters, so if the chain marriage gets into more detail later on, I can't make any judgment on that.

I have. If you'd prefer I not color/spoil anything I can leave out any information from later in the book. (I try to avoid spoilers, but I believe on previous discussions I've corrected misconceptions/incorrect assumptions if they're contradicted later in the book.)

From what I gathered it was the second most senior she slept with instead, and only because she was biologically related to the most senior husband.

I don't recall the 'rank' of the husband she slept with. I'm inclined to believe he was somewhat junior based on my gut, but I think they flesh that out within the next few chapters.

I've read Door To Summer, Space cadet, Red Planet, Farmer in the sky, and Between Planets, which i gather were aimed at a younger audience and thus avoid sexual discussions.

I've heard that Starship Troopers was his last YA/first adult novel, though based on the one book I read published before that (The Puppet Masters), he was at least moving away from YA well before Starship Troopers.