Truly declarative language?

It looks like you just want to make Z store a function instead of a value. In C#:

var X = 10;    // define and assign two variables
var Y = 20;

Func<int> Z = () => X + Y;  // declare a formula that uses these two variables

Console.WriteLine(Z());

X = 50;     // change one of the input variables

Console.WriteLine(Z());

So the equivalent of your ?-prefix syntax is a ()-suffix, but otherwise it's identical. A lambda is a "formula" in your terminology.

Behind the scenes, the C# compiler builds almost exactly what you presented in your C# conceptual example: it makes X into a field in a compiler-generated class, and allocates an instance of that class when the code block is entered. So congratulations, you have re-discovered lambdas! :)

Any Constraint Programming system will do that for you. Examples of CP systems that have an associated language are ECLiPSe, SICSTUS Prolog / CP package, Comet, MiniZinc, ...