Hey Purdy. Welcome to the thread.
1. Yep, after his bike got demolished in the accident he had, he wanted to buy a new one. He found some super awesome bike (or just the engine?) he wanted to have at all costs and because he lacked the money he proposed to Hilde so he could use their joined assets. Of course he didn't tell her that. He told her he'd swear off women and alcohol but in the end Hilde married him because he would cut "the stupid braid."
Don't look at me, I didn't write it.
2. I don't know what the significance of the music box is yet but what I know is that there is (and has been for years now) an EW novelization, which features said music box. I don't know if Sumizawa wrote that novel (doubt it) but so far he has incorporated it and Preventer 5 the way they originally are, without silly additions like obscure kinships or what have you. For all intents and purposes the music box is probably a memento of Leia Barton which she gave to her daughter before she died or something. Or perhaps they incorporated it to add to her little speech about life being like an endless waltz (I think it plays that song).
Now how and why it ended up in Heero's hands, I have no idea.
3. Nope, "Interlude of the Heart" Part 1 is all we have right now. Part 2 will be available whenever Ace is out and Deacon feels like it. But to be honest, the chapter wasn't that bad. In fact, I find it to be one of the best of the entire novel. Less crazy, more awesome.
4. lol His name can be translated as either Marticus or Martix. I don't have the time right now to summarize all their back stories but as far as I can see the gundam wikia covers the most important aspects.
As for what the significance of all the back story is... *shrug* I guess we'll have to wait and see?
5. I'm very fuzzy on the details because I honestly can't be bothered to care much, so you should probably take the others' word for it but from what I remember it was more fighting between Heero and co. and Relena's followers, which ended in Heero convincing his folks to surrender so that he could be captured and brought to Relena to kill her. And let's not forget that he shed manly tears at the thought of it.
Hope that helps a little.