I made a Jell-O last night and it turned out too sweet for me. Is it doable to increase the amount of water used but the same amount of Jell-O powder? So instead of 1 cup of hot water, 1 cup of cold water, and 1 Jell-O pack, it would be something like 2 cups of hot water, 2 cup of cold water, and 1 Jell-O pack. Or would doing this result in the Jell-O not solidifying?
If you add too much water your jello (jelly in some parts of the world) will not solidify. What I would suggest is looking for a non-sweetened gelatin mix and adding less sugar to it. Alternatively you could get gelatin sheets or powder and make your own from scratch. It's not that hard to do and you get exactly what you want.
I am a big fan of plain gelatin powder. It has no sugar, no coloring, and no flavoring. If you want to use a regular flavored jello as your base you can easily use 1/2 flavored jello and 1/2 unflavored gelatin (measure by water or liquid), and get a milder version. You can also make jello from fruit juice and unflavored gelatin without adding any sugar getting all your sweetness from the juice. I do this all the time using the juice from canned pineapple (I get the type that is canned in pineapple juice) and fill it with the pineapple from the same can. Total ingredient list: one #10 can of pineapple, 6 or 7 packets of gelatin (depending on my mood), water (again mood dependant). The result is sweet, but not too sweet.