Can you spend more than one String at once on the same effect?
Like, can I spend 2-strings to do +2harm?
Good question. No, you can't.
1.
The process of turning the characters' initial Strings (Hx) into backstory for the game could be an official part of the first session. One way of doing this is to have a good discussion about it before starting play; another way would be to have flashbacks between different characters when the time felt right.
Strings and backstory feel really important to Monsterhearts.
Neat idea, to establish them throughout play. I'll have to ruminate on this idea further, but my initial reaction is, "Sweet! Do it, and tell me how it goes!"
Interestingly,
your backstory is a simple idea that I tacked on while I was laying out the character pages. I thought to myself, "Gee, characters don't really have enough stuff to them, do they? Maybe I'll put in a thing so that they start with some Strings."
2. In the 'Fires' section, it might be worth putting in a reminder that the MC can use the characters' origins (that they selected in their playbooks) as inspiration for fires.
Good idea.
I'm still waffling on how Fires work, and wants/means work. You'll probably note that it doesn't seem to gel/coalesce like the rest of the game does.
Figuring out how Threats work in this game is my stumbling block, f'sure.
That said: your idea is brilliant, and that's a big part of why origins are in the game.