I think the idea with the Cool Whip is that as long as you stick with the serving size, it's a minimal amount of both trans fats and sugar. I did find that the Wal*Mart brand version of the FF Cool Whip had some slightly better ingredients (or maybe a better mix of them?).
You can always make your own with whipping cream and splenda/SF syrups, as you've been doing. If you freeze it, however, it tends to become pretty hard, and it doesn't last too long in the fridge. I don't remember what's in Whip It, but it's a whipping cream stablizer that might help you keep some in the fridge for longer.
Sounds kind of weird, I guess, but what about using FF sour cream with splenda? I used to eat strawberries with sugar and sour cream when I was a kid and it's totally delicious!