how to make fruit-juice flavored gelatin firm vs watery?

You could try adding more gelatin to the mix and see how it turns out, though I wouldn't be surprised if your already firm half just ends up too hard then. I think if you end up with a different consistency throughout your jelly then the gelatin probably didn't dissolve completely/properly. Try to fully dissolve it in the hot water first and then add the fruit juice, maybe that'll help.

I'd also try different kinds of juice to see if you get similar results.

Maybe you can give something like Agar Agar (it's some kind of algae powder) a try. In my experience, it works very well as a substitute to regular gelatin. It can be tricky though. If you mess up the ratios, you might just end up with something resembling cement made out of jelly.

First of all, adding more gelatin is not something I would try. If the problem were too little gelatin, you would have a result that is uniformly too-soft or even liquid, not a difference between top and bottom. Also, adding more "just to be sure" is a bad idea in general, since extra gelatin makes for a rather unpleasant rubbery texture and also reduces aroma.

You don't give much information to go on, but I will assume that you meant

I use hot water mixed with room-temperature juice (making sure proportions of liquid to powder are as needed), mix very thoroughly, and set it overnight in fridge.

to be literally your full process. If that's so, then you didn't properly use the gelatin, and it is normal that you are seeing weird results.

The proper way to use gelatin is to bloom it.

1. Take the powder and let it soak in some tap water - 5% to 10% of the final liquid volume is common. You will recognize it's done when it has swollen up and formed a single wet mass, you can barely recognize the separate grains
2. Heat your juice to room temperature or slightly above
3. Stir in the bloomed gelatin into the juice
4. Warm up the whole mixture, without letting it get too hot, not even simmer. It should be between 50 C and 70 C when you take it off the heat. The gelatin will have dissolved before reaching the temperature, and you will have a uniform liquid.
5. Pour the mixture into a mold and put it in the fridge for several hours, best overnight.

This is the basic process for gelatin. It has a few variations (e.g. when you are making a whipped-cream mousse), but for your case, this is what will give you a proper juice-based dessert.