Is grammar that describes an equation in prefix (Polish) notation always unambiguous?
Your question is easy to answer. Take your grammar, and add the following rules:
$$
\begin{align*}
&amp;S \to T \mid \mathit{Expr} \\
&amp;T \to \mathit{Expr}
\end{align*}
$$

Your particular grammar, however, does seem unambiguous.

Here is how I would try to prove it. First, show the expressions are self-delimiting: no expression is a prefix of another one.

Now, when trying to parse an expression, you can determine which rule should be applied first by looking at the first symbol. In the interesting cases, you then have to partition the input into two expressions. The self-delimiting property should make this possible in a unique way.