You aren't going to find any magic solutions. The best implementation is the one that is least likely to become unsuitable as you add more to your program. If you don't know which one that is, we can't help you without learning more about your program than you do, which is silly. If you pick an implementation that later becomes unsuitable, it's not the end of the world; just refactor it. Bottom line: just do it.

That's already on the to-do list. The scoring will attempt to measure your skill based on wins and losses. If there were anything based on kills, players would spend less time playing the game the way it's intended in order to artificially increase their rating.

I do not understand people who make video games but don't play video games. If you program for the sake of technology, why not make a simulation or tech demo?

I may not entirely understand where you're coming from, but if I am distracted by something I can't let go, I find it helps to write it down in a spiral bound notebook I keep for the purpose. After writing it down, I know I will come back to it later, so I don't have to worry about forgetting it.

You are deprived if a video game has never elevated your heartrate and made you feel adrenaline.

If you're talking about action games, then you're not entirely correct; action games are light exercise. Sure, sports are better for you, but games are still better exercise than reading, watching TV, or doing deskwork. Also, with both hands tied up, you can't munch on potato chips.

There is no ideal API. They all have advantages and disadvantages. I can suggest, however, the Java2D engine, which I used to make Battlement.

It's probably a browser problem. I would ask your customers what browser they're using if I were you. It looks like the applet is not being repainted after it is invalidated. Calling the applet's repaint() method after it loads might fix the login screen. As for the loading screen, consider using this kind of loading screen instead.