1. Yes
2. Yes. I created a new layout using my usual workflow, which is not to make huge changes on anything. I first make a duplicate, then make the changes. It saves me from shooting myself in the foot when I bargle things up. I can always delete the mess, and be back where I was when I started screwing things up. In this case, I deliberately left the original layout in case you ever want a slide without the scoreboard.
3. Technically, the scoreboard only shows on the base. The layers are set to allow the scoreboard from the base show through. It'd less work than to create a layout for layers, too.
4. Yes. however you increment score, the triggers on the master will take care of the scoreboard.
The less than value 9 is due to the fact that I believe when you make something fool-proof, the universe makes a smarter fool. Actually, incrementing a variable is always dangerous. What happens if someone tries the question twice, and the score becomes 9? There are frequently ways that they can be incremented beyond what you expect, and bad things can happen. In this specific instance, I can't think of how, or what problems it could cause, but why take chances?
5. There are two important components to complicated operations in SL; your general knowledge of what it can be made to do and how, and how well you define your problems. I learned a lot about SL by frequenting (practically stalking) the forum, reading questions and answers I wasn't even interested in, just to learn what I could about the program. I looked for unanswered questions to see if I could find answers, and truthfully, I made it somewhat of a hobby.
Problem definition is the most important. Far too many of us think describing what it looks like to the learner is adequate problem definition. What we need is to know specifically, clearly, and accurately what do I need, and how should the computer do it, one step at a time. What tiny, single action steps does the computer need to take, and in what order. The smaller you can get the details, the more likely you are to have a chance of implementing them.
Also, you might check out YouTube. I've heard that there is a bunch of videos there, but I can't vouch for their value.