An easier approach would be to use the reduced Mayer-Vietoris sequence (which exists in arbitrary homology theories) as follows:
We can write $X\vee Y$ as a union of the two open subsets $U=X\cup N$ and $V=Y\cup N$. Note that $U$, respectively $V$, deformation retract onto X, respectively $Y$. Moreover, the intersection $U\cap V$ deformation retracts onto a point.
Thus, the Mayer-Vietoris sequence corresponding to the open cover $X=U\cup V$ reads:
$$\cdots \to \underbrace{\widetilde H_i(\text{pt})}_{=0}\to \widetilde H_i(X)\oplus \widetilde H_i(Y)\to \widetilde H_i(X\vee Y)\to \underbrace{ \widetilde H_{i-1}(\text{pt})}_{=0}\to\cdots$$
and the middle map is induced by the inclusions $X,Y\hookrightarrow X\vee Y$. This provides the desired isomorphism.
Let us first understand the isomorphism $\mathcal{H}_n(X) \cong \widetilde{\mathcal{H}}_n(X) \oplus \mathcal{H}_n(*)$. You define reduced homology groups by
$$\widetilde{\mathcal{H}}_n(X) = \ker (c_* : \mathcal{H}_n(X) \to \mathcal H_n(*)) \tag{R}$$
where $c : X\to *$ denotes the unique map to the one-point space $*$.
Clearly each $f : X \to Y$ has the property $f_*(\widetilde{\mathcal{H}}_n(X)) \subset \widetilde{\mathcal{H}}_n(Y)$, i.e. we get an induced homomorphism $\widetilde f_* : \widetilde{\mathcal{H}}_n(X) \to \widetilde{\mathcal{H}}_n(Y)$ which satisfies $i_Y \widetilde f_* = f_* i_X$. It is easy to see that this construction gives us functors $\widetilde{\mathcal{H}}_n$.
By definition $\text{(R)}$ we get the short exact sequence
$$0 \to \widetilde{\mathcal{H}}_n(X) \stackrel{i}{\hookrightarrow} \mathcal{H}_n(X) \stackrel{c_*}{\to} \mathcal H_n(*) \to 0 \tag{E}$$
Note that $c_*$ is an epimorphism because it has a right inverse. In fact, each map $j : * \to X$ has the property $c_* j_* = id$. The latter also shows that $\text{(E)}$ is a split short exact sequence.
Therefore each $j : * \to X$ induces an isomorphism
$$j_\# : \widetilde{\mathcal{H}}_n(X) \oplus \mathcal H_n(*) \to \mathcal H_n(X), j_\#(a,b) = i(a) + j_*(b) \tag{S}$$
More generally we could take any right inverse $\iota : \mathcal H_n(*) \to \mathcal H_n(X)$ of $c_*$ to produce an isomorphism $$\iota_\# : \widetilde{\mathcal{H}}_n(X) \oplus \mathcal H_n(*) \to \mathcal H_n(X), \iota_\#(a,b) = i(a) + \iota(b) \tag{S'}$$
In general not all of these $\iota$ will have the form $\iota = j_*$ for some $j$. Anyway, the assocation $\iota \mapsto i_\#$ described in $\text{(S')}$ establishes a bijection from the set of right inverses $\iota$ of $c_*$ to the set of isomorphisms $\widetilde{\mathcal{H}}_n(X) \oplus \mathcal H_n(*) \to \mathcal H_n(X)$.
For a path-connected $X$ all maps $j: * \to X$ are homotopic, i.e. $j_*$ does not depend on the choice of $j$. We therefore get a canical isomorphism $J = J_X : \widetilde{\mathcal{H}}_n(X) \oplus \mathcal H_n(*) \to \mathcal H_n(X)$ which has the form $J =j_\#$ for any choice of $j$.
On the full subcategory of path-connected spaces the above $J_X$ form a natural isomorphism. This follows from
Lemma. Let $Y$ be path connected, $f : X \to Y$ and $j_X : * \to X$. Then $J_Y (\widetilde f_* \oplus id) = f_*(j_X)_\#$.
Proof. We have $J_Y = (j_Y)_\#$ with $j_Y = f j_X$ and we get
$$J_Y(\widetilde f_* \oplus id)(a,b) = (j_Y)_\# (\widetilde f_* \oplus id)(a,b) = (j_Y)_\#(\widetilde f_*(a),b) = i_Y \widetilde f_*(a) + (j_Y)_*(b) \\= f_*i_X(a) + (fj_X)_*(b) = f_*i_X(a) + f_*(j_X)_*(b) = f_*(i_X(a) + (j_X)_*(b)) = f_*(j_X)_\#(a,b) .$$
For the general case we have
Theorem. There exists a natural isomorphism $t_X : \widetilde{\mathcal{H}}_n(X) \oplus \mathcal H_n(*) \to \mathcal H_n(X)$ on the category of all toplogical spaces if and only if $\mathcal H_n(*) = 0$. NB: $\mathcal H_n(*) = 0$ does not imply that $\mathcal H_n(X) = 0$ for all $X$.
Proof. If $\mathcal H_n(*) = 0$, then $\widetilde{\mathcal{H}}_n(X) = \mathcal H_n(X)$. We can take $t_X = id$.
Now let $\mathcal{H}_n(*) \ne 0$. Then $\widetilde{\mathcal{H}}_n(*) = 0$. Consider $Y = \{1,2\}$ with the discrete topology. We have two maps $f_i : * \to Y$ given by $f_i(*) = i$. The excision axiom gives us an isomorphism $\phi : \mathcal{H}_n(Y) \to \mathcal{H}_n(*) \oplus \mathcal{H}_n(*)$ which has the property that $\phi (f_i)_*$ is the inclusion in the $i$-th summand. Thus $(f_1)_* \ne (f_2)_*$. If there would exist a natural isomorphism $t_X$, we would get two commutative diagrams
\begin{CD}
\mathcal{H}_n(*) @>(f_i)_*>> \mathcal{H}_n(Y)\\
@Vt_*VV @VVt_YV \\
0 \oplus \mathcal{H}_n(*) @>\widetilde{(f_i)}_*\oplus \text{id}>> \widetilde{\mathcal{H}}_n(Y) \oplus \mathcal{H}_n(*)
\end{CD}
But this is impossible because $t_Y(f_1)_* \ne t_Y(f_2)_*$, but $\widetilde{(f_1)}_* = \widetilde{(f_2)}_* = 0$.
Best Answer
Since $\omega_*$ is linear, we have $\omega_*([\sigma_1] \oplus [\sigma_2]) = \omega_*([\sigma_1] \oplus 0) + \omega_*(0 \oplus [\sigma_2])$.
Let $i_1 : G \to G \vee G$ be the inclusion of the first factor in the wedge sum. Note that $[\sigma] \oplus 0 = (i_1)_*([\sigma])$, and that $\omega \circ i_1$ is the identity of $G$ (assuming that the base point of $G$ is the unit of $m$). It follows that $\omega_*([\sigma] \oplus 0) = (\omega \circ i_1)_* ([\sigma]) = [\sigma]$. Of course, the same holds for $\omega_*(0 \oplus [\sigma])$, so in the end, you get $$\omega_*([\sigma_1] \oplus [\sigma_2]) = [\sigma_1] + [\sigma_2].$$