Method 1: Group action
Note that $X=(S^2\times S^1)/\Bbb Z_2$, where the action of $\Bbb Z_2$ is generated by $(x,z)\mapsto (-x,-z)$. We can also rewrite $X$ as a quotient space of $S^2\times\Bbb R$ using the fact that $S^1\approx \Bbb R/\Bbb Z$, then the equivalence relations are $(x,y)\sim(x,y+n)$ for $n\in\Bbb Z$ (given by the circle) and $(x,y)\sim (-x,y+1/2)$. Combining these two pieces, we see that the equivalence relation is defined by $(x,y)\sim((-1)^nx,y+n/2)$. Therefore, $X=(S^2\times\Bbb R)/\Bbb Z$, where the action of $\Bbb Z$ is generated by $(x,y)\mapsto(-x,y+1/2)$.
Note that this group action is free, so we have $\pi_1(X)\cong\Bbb Z$. According to Hurewicz, $H_1(X;\Bbb Z)\cong \Bbb Z$ as well. Since this $\Bbb Z_2$-action on $S^2\times S^1$ is orientation reversing, $X$ is non-orientable, so $H_3(X;\Bbb Z)\cong 0$ and the torsion of $H_2(X;\Bbb Z)$ is $\Bbb Z_2$.
To determine the rank of $H_2$, we use the fact that the quotient map $q:S^2\times S^1\to X$ is a double cover, so $2\chi(X)=\chi(S^2\times S^1)=0\implies \chi(X)=0$. On the other hand,
$$\chi(X)=b_0-b_1+b_2-b_3=1-1+b_2-0=b_2$$
where $b_i$'s are betti numbers. This shwos that the rank of $H_2$ is $0$.
In conclusion,
$$H_k(X;\Bbb Z)\cong\begin{cases}\Bbb Z & k=0,1\\ \Bbb Z_2&k=2\\ 0 &\text{ otherwise}\end{cases}$$
Method 2: Cellular homology
The cell structure of $S^2$:
- two $0$-cells $e_N^0, e_S^0$ (one at the north pole and another one at the south pole);
- two $1$-cells $e_\alpha^1$ (oriented from the south pole to north pole) $ e_\beta^1$ (oriented from the north pole to the south pole);
- two $2$-cells $e_\alpha^2$, $e_\beta^2$.
The cell structure of $S^1$ is constructed in the same way as $S^2$.
- two $0$-cells $f_N^0, f_S^0$;
- two $1$-cells $f_\alpha^1, f_\beta^1$.
The reason to choose such non-standard cellular decomposition with 2-fold symmetry is that we want to obtain another cell structure when we descends to the quotient space $X$.
The cell structure $S^2\times S^1$ should be obvious from the construction above, i.e., four $3$-cells, eight $2$-cells, eight $1$-cells, and four $0$-cells. Now, consider the equivalence relation $(x,z)\sim (-x,-z)$, we see that cells are collapsed in pairs. For instance, $e_\alpha^1\times f_N^0$ is identified with $e_\beta^1 \times f_S^0$, so we have the following cellular chain complex for $X$.
$$0\to\Bbb Z^2\overset{\partial_3}{\to}\Bbb Z^4\overset{\partial_2}{\to}\Bbb Z^4\overset{\partial_1}\to\Bbb Z^2\to 0$$
Consider the one $e_\alpha^1\times f_N^0$, its image under the boundary map is $\partial_1(e_\alpha^1)\times f_N^0=(e_N^1-e_S^1)\times f_N^0$. We don't have to consider its "partner" $e_\beta^1\times f_S^0$ because they are identified in the quotient space. Similarly, one can argue that $\partial_1(e_\beta^1\times f_N^0)=(e_S^0-e_N^0)\times f_N^0=-\partial_1(e_\alpha^1\times f_N^0)$ and that $\partial_1(e_N^0\times f_\alpha^1)=e_N^0\times(f_N^0-f_S^0)=-\partial_1(e_N^0\times f_\beta^0)$. This shows that
- $\ker(\partial_1)=\langle(e_\alpha^1+e_\beta^1)\times f_N^0, e_N^0\times (f_\alpha^1+f_\beta^1),e_N^0\times f_\alpha^1+e_\beta^1\times f_N^0\rangle$
- $\operatorname{im}(\partial_1)=\langle(e_N^0-e_S^0)\times f_N^0\rangle$. (compute $e_\alpha^1\times f_N^0, e_\beta^1\times f_N^0, e_N^0\times f_\alpha^1, e_N^0\times f_\beta^1$)
Similar arguments can be repeated for $\partial_2$ and $\partial_3$ using the formula of cellular boundary map. Note that we need to refer back to the equivalence relation to see the relationship between $\ker\partial_{k}$ and $\operatorname{im}\partial_{k+1}$.
- $\ker(\partial_2)=\langle e_\alpha^2\times (f_N^0-f_S^0), (e_\alpha^1+e_\beta^1)\times f_\alpha^1\rangle$; (compute $e_\alpha^2\times f_N^0, e_\alpha^2\times f_S^0, e_\alpha^1\times f_\alpha^1, e_\beta^1\times f_\alpha^1$)
- $\operatorname{im}(\partial_2)=\langle(e_\alpha^1+e_\beta^1)\times f_N^0, (e_N^0-e_S^0)\times f_\alpha^1+e_\alpha^1\times (f_N^0-f_S^0)\rangle$;
- $\ker(\partial_3)=0$;
- $\operatorname{im}(\partial_3)=\langle (e_\alpha^1+e_\beta^1)\times f_\alpha^1+e_\alpha^2\times(f_N^0-f_S^0),(e_\alpha^1+e_\beta^1)\times f_\alpha^1+e_\alpha^2\times(f_S^0-f_N^0)\rangle$ (compute the image of $e_\alpha^2\times f_\alpha^1$ and $e_\alpha^2\times f_\beta^1$)
If we compute the quotient groups carefully, then we actually get the same answer as using method 1.
Your computation is correct. One thing to note, which you probably understand, is that $H_1(X,\partial X)=0$ follows not just because $H_0(\partial X)\longrightarrow H_0(X)$ is an isomorphism, but because also the map coming to $H_1(X,\partial X)$ is zero by the surjectivity of $f_\ast$ that you showed above.
Homology is not as simple as counting cells, unfortunately. After all, any CW-complex can be seen as built by attaching cells to a point. However, amounts of cells don't determine the ("relative") homology groups $H_{\bullet}(X, pt) \simeq \tilde{H}_{\bullet}(X)$. For example, the homology of the projective plane, which is made from a $1$-cell and a $2$-cell, is not $\mathbb{Z}$ but $\mathbb{Z}/2$ in dimension $1$.
Best Answer
Let $t$ denote the antipodal map on $S^n$. There is a simple CW structure for $S^n$ compatible with $t$, which we define inductively by $S^0=\{*,*t\}$, for a point $*$, and $$S^n=S^{n-1}\cup_\phi e_n\cup_\psi e_nt,$$ where $e_n$ is an $n$-cell and $\phi,\psi$ are identifications of the boundaries of $e_n$ and $e_nt$ with $S^{n-1}$.
Intuitively, we regard each sphere as the equator of the next, and glue on a couple of hemispheres.
The boundary maps $d$ are given by $$de_r=e_{r-1}(1+(-1)^rt),\qquad de_0=0.$$
To understand the sign $(-1)^r$ here, note if $r$ is odd then $t$ preserves the orientation of $S^{r-2}$, so the boundary of $e_r$ must be the difference $e_{r-1}-e_{r-1}t$ in order to be closed. Conversely if $r$ is even, then $t$ reverses the orientation of $S^{r-2}$, so the boundary of $e_r$ must be the sum $e_{r-1}+e_{r-1}t$ in order to be closed.
We may take the tensor product of the resulting chain complexes for $S^n$ and $S^m$, to obtain a chain complex for $S^n\times S^m$.
Here the degree $r$ chain group is generated by \begin{eqnarray*}e_a\otimes e_{r-a},\\ e_at\otimes e_{r-a},\\ e_a\otimes e_{r-a}t,\\ e_at\otimes e_{r-a}t.\end{eqnarray*} for $a=0,\cdots, r|a\leq n, r-a\leq m$. The differential is given by \begin{eqnarray*}d(e_i\otimes e_j)&=&e_{i-1} \otimes e_j\\ + &(-1)^i& e_{i-1}t \otimes e_j\\+&(-1)^i& e_{i} \otimes e_{j-1}\\&(-1)^{i+j}& e_{i} \otimes e_{j-1}t. \end{eqnarray*}
That is we use Leibniz's rule for tensor products:$$d(a\otimes b)=da\otimes b+ (-1)^{{\rm deg}(a)} a\otimes db.$$
Finally, we obtain a chain complex for $$X=(S^n\times S^m)/\langle t\rangle,$$ by taking our chain complex and identifying $$e_i\otimes e_j\sim e_it\otimes e_jt,\qquad e_i\otimes e_jt\sim e_it\otimes e_j.$$
We may write the boundary maps $d$ as matrices, with respect to the basis $\ldots e_a\otimes e_b, e_at\otimes e_b, e_{a+1}\otimes e_{b-1},\ldots$.
The homology groups of $X$ may then be computed as an exercise in linear algebra. For example, if $0<<m<<n$ and $n,m$ both odd I get that the non-zero homology groups are:
\begin{eqnarray*} H_0(X)\cong H_{n+m}(X)&\cong& \mathbb{Z},\\ H_m(X)\cong H_n(X)&\cong&\mathbb{Z},\\ H_{2i-1}(X)\cong H_{n+m-2i}&\cong& \mathbb{Z}/2\mathbb{Z}, \qquad{\rm for} \,\,i=1,\cdots,(m-1)/2.\end{eqnarray*}
I suggest that you do this calculation yourself, as it is quite fiddly and I may have made an error. However the above answer has the symmetries one would expect. If you require help, I can give you the matrices.