Some of the details can be removed/abstracted-away: let $G$ be a compact topological group (need not be Lie...), and $V$ a finite-dimensional (real?) representation space for $G$. Then $V$ admits a $G$-invariant inner product. Specifically, given any inner product $\langle,\rangle$ on $V$, the averaged inner product
$$
\langle v,w\rangle' \;=\; \int_G \langle gv,\,gw\rangle\;dg
$$
with Haar measure $dg$ giving $G$ total mass $1$, is $G$-invariant.
The proof you've sketched, with various explicit details suppressed, succeeds in this context, simply by "changing variables" in the integral. In fact, it succeeds for possibly infinite-dimensional Hilbert spaces $V$, using a bit of elementary functional analysis.
The change-of-variables trick you were hesitant about is exactly the invariance of the functional $f\to \int_G f(g)\,dg$, namely, that for all $h\in G$ we have $\int_G f(gh)\,dg=\int_G f(g)\,dg$. And this invariance is an equivalent of invariance of the Haar measure.
Let's assume that $df$ is left-invariant. Thus
$$ L_g^* df = df \Rightarrow d(L_g^* f) = df.$$
Since $G$ is connected, this implies that $L_g^* f - f$ is constant. So
$$ L_g^* f = f + C(g)\Rightarrow f(gh) = f(h) + C(g) , \ \ \ \forall h, g\in G.$$
Now assume that $f(e) = 0$ (this can be done by adding a constant to $f$, which does not change $df$). Then with $h = e$,
$$ f(g) = f(e) + C(g) \Rightarrow f(g) = C(g).$$
Thus $f(gh) = f(h)+ f(g)$: that is, $f$ is a homomorphism. So we have $(1)\Rightarrow (4)$. $(2)\Rightarrow (4)$ is similar.
On the other hand, if $f$ is a smooth homomorphism, then for each fixed $g$,
$$ L_g^* f = f + f(g)\Rightarrow d (L_g^* f) = df\Rightarrow L_g^* df = df.
$$
So $df$ is left-invariant. Similarly one can show $(4)\Rightarrow (2)$.
Best Answer
Let $X_1, \cdots, X_n \in T_e G$. Using $i_* X = -X$ (see here), we have
$$ (i^* \omega)_e (X_1, \cdots, X_n) = \omega_e (i_* X_1, \cdots, i_*X_n) = (-1)^n \omega_e (X_1, \cdots, X_n),$$
thus $(i^*\omega)_e =(-1)^n \omega_e$. Since both $\omega, i^*\omega$ are left invariant, we have $i^*\omega = (-1)^n \omega$.