I would add to the previous list of "applications" of diffeology in ordinary differential geometry the theorem about *Homotopic invariance of De Rham cohomology* (§6.88 of [Diffeology][1]). In brief:

**Theorem**: *Let $X$ and $X'$ be two diffeological spaces. Let $f_1$ and $f_2$ be two homotopic smooth maps from $X$ to $X'$. Let $\alpha'$ be a closed $k$-form on $X'$, then $f_2^*(\alpha')$ is cohomologous to $f_1^*(\alpha')$.*

THe diffeological proof uses the Chain-Homotopy operator 

$$
K : \Omega^p(X) \to \Omega^{p-1} (\mathrm{Paths}(X)), \ \text{that satisfies} \ K \circ d + d \circ K = \hat 1^* - \hat 0^*,
$$

where $\mathrm{Paths}(X) = C^\infty({\mathbf R}, X)$ is the set of smooth paths in $X$, and for all $\gamma \in \mathrm{Paths}(X)$, $\hat t(\gamma) = \gamma(t)$.

**Proof**: For all $x \in X$ let:

$$
\phi(x) = [s \mapsto f_s(x)],  \ \text{then} \ \phi \in C^\infty\big( X,\mathrm{Paths}(X')\big).
$$

Apply $\phi^*$ to the identity above satisfied by $K$, evaluated on $\alpha'$ 
$$
\phi^*[K d\alpha' + d K\alpha'] = \phi^*(\hat 1^*(\alpha')) - \phi^*(\hat 0^*(\alpha')).
$$
Since $d\alpha' = 0$ and $\hat t \circ \phi = f_t$ we get
$$
f_2^*(\alpha') = f_1^*(\alpha') + d\beta, \ \text{with} \  \beta = \phi^*(K\alpha').
$$
Therefore, $f_2^*(\mathrm{class}(\alpha')) = f_1^*(\mathrm{class}(\alpha'))$.$\square$

This general theorem applies in particular for $X$ and $X'$ manifolds since the category of manifolds is a full subcategory of diffeological spaces. This is an example where diffeology helps in a proof of a classical differential geometry theorem, using a shortcut thru a space which is not a manifold: the space $\mathrm{Paths}(X)$. And as we see it was important to have a good definition of differential forms on every diffeological space, whatever they are.

By the way the Chain-Homotopy operator is used also in *Symplectic Diffeology* for the definition and the generalisation of [The Moment Map in Diffeology][2]. You will find in that Memoir a few of examples of application of diffeology in symplectic geometry.


  [1]: https://www.icloud.com/iclouddrive/08bIgfV5NT8F8oBo-Ipqoz1kg#Diffeology
  [2]: https://bookstore.ams.org/memo-207-972