I had cause to think about this exercise recently so I thought I’d write an answer. I think Jim’s answer is quite sufficient but as you seem to want more details I’ll provide them here. I am well aware that you know some of the following but I’m writing a complete solution for the benefit of future readers.

I’ll use a slight generalisation of the construction given in Cabanes—Enguehard. I’ll write $F : G \to G$ for a Frobenius endomorphism of $G$. Assume $T$ is a torus equipped with a Frobenius endomorphism, which we also denote by $F : T \to T$, and further let us assume that we have a closed embedding $\pi : Z(G) \hookrightarrow T$ which is defined over $\mathbb{F}_q$. The direct product $G \times T$ inherits a natural Frobenius endomorphism given by $F \times F$ and the subgroup $\Delta_{\pi}(Z(G)) = \{(z,\pi(z)^{-1}) \mid z \in Z(G)\}$ is an $F$-stable closed subgroup of $G \times T$. We denote by $G \times_{Z(G)} T$ the quotient group $(G\times T)/\Delta_{\pi}(Z(G))$.

Now assume $\sigma : G \hookrightarrow H$ is a regular embedding. By assumption we have $H = \sigma(G)Z(H)$ and so the morphism $G \times Z(H) \to H$ given by $(g,z) \mapsto \sigma(g)z$ is surjective. The kernel of this map is clearly $\Delta_{\sigma}(Z(G))$ so we have an induced bijective morphism of varieties $\gamma : G \times_{Z(G)} Z(H) \to H$. Your real question seems to be whether or not $\gamma$ is an isomorphism. It is worthwhile thinking about this as one wants to rule out situations like the natural projection map $\mathrm{SL}_p(K) \to \mathrm{PGL}_p(K)$ in characteristic $p$, which fails to be an isomorphism. However, this essentially doesn’t happen because our derived subgroups are assumed to be isomorphic.

By Corollary 5.3.3 of Springer’s LAG we need only check that the differential $d_e\gamma$ at the identity is bijective. First, let’s note that we have

$$T_e(H) = T_e(H_{\mathrm{der}}) + T_e(Z(H)),$$

where $H_{\mathrm{der}} \leqslant H$ is the derived subgroup of $H$. This is easily seen from the fact that $T_e(H)$ is the sum of a maximal toral subalgebra together with the root spaces. Note, however, that this sum need not be direct. For instance, think of $\mathrm{GL}_p(K)$ in characteristic $p$. Although it is not direct it implies the surjectivity of our map $\gamma$ because $\sigma$ maps the derived subgroup $G_{\mathrm{der}} \leqslant G$ isomorphically onto $H_{\mathrm{der}}$ and we similarly have

$$T_e(G \times_{Z(G)} Z(H)) = T_e(G_{\mathrm{der}} \times_{Z(G)} 1) + T_e(1 \times_{Z(G)} Z(H)).$$

This is really highlighting that the problems essentially arise at the derived subgroup.

As you observed, with this observation the exercise becomes fairly easy. Assume $\sigma : G \to G’$ and $\tau : G \to G’’$ are closed embeddings then we by $T$ the torus $Z(G’) \times Z(G’’)$. This inherits a natural Frobenius endomorphism from $G’$ and $G’’$ and we have a closed embedding $Z(G) \hookrightarrow T$ given by $z \mapsto (\sigma(z),\tau(z))$ which is defined over $\mathbb{F}_q$, so we can form the group $G’’’ = G \times_{Z(G)} T$. We have regular embeddings $G’ \to G \times_{Z(G)} Z(G’) \to G \times_{Z(G)} T$ and $G’’ \to G \times_{Z(G)} Z(G’’) \to G \times_{Z(G)} T$ where the first maps are the isomorphisms constructed above. It is then easy to see that we may take your group $G’’’$ to be the group $G \times_{Z(G)} T$.

I had cause to think about this exercise recently so I thought I’d write an answer. I think Jim’s answer is sufficient but as you seem to want more details I’ll provide them here. I am aware that you know some of the following but I’m writing a complete solution for the benefit of future readers.

I’ll use a slight generalisation of the construction given in Cabanes—Enguehard. I’ll write $F : G \to G$ for a Frobenius endomorphism of $G$. Assume $T$ is a torus equipped with a Frobenius endomorphism, which we also denote by $F : T \to T$, and further let us assume that we have a closed embedding $\pi : Z(G) \hookrightarrow T$ which is defined over $\mathbb{F}_q$. The direct product $G \times T$ inherits a natural Frobenius endomorphism given by $F \times F$ and the subgroup $\Delta_{\pi}(Z(G)) = \{(z,\pi(z)^{-1}) \mid z \in Z(G)\}$ is an $F$-stable closed subgroup of $G \times T$. We denote by $G \times_{Z(G)} T$ the quotient group $(G\times T)/\Delta_{\pi}(Z(G))$.

Now assume $\sigma : G \hookrightarrow H$ is a regular embedding. By assumption we have $H = \sigma(G)Z(H)$ and so the morphism $G \times Z(H) \to H$ given by $(g,z) \mapsto \sigma(g)z$ is surjective. The kernel of this map is clearly $\Delta_{\sigma}(Z(G))$ so we have an induced bijective morphism of varieties $\gamma : G \times_{Z(G)} Z(H) \to H$. Your real question seems to be whether or not $\gamma$ is an isomorphism. It is worthwhile thinking about this as one wants to rule out situations like the natural projection map $\mathrm{SL}_p(K) \to \mathrm{PGL}_p(K)$ in characteristic $p$, which fails to be an isomorphism. However, this essentially doesn’t happen because our derived subgroups are assumed to be isomorphic.

By Corollary 5.3.3 of Springer’s LAG we need only check that the differential $d_e\gamma$ at the identity is bijective. However as both tangent spaces have the same dimension we need only check surjectivity, which follows from the fact that

$$T_e(H) = T_e(H_{\mathrm{der}}) + T_e(Z(H)),$$

where $H_{\mathrm{der}} \leqslant H$ is the derived subgroup of $H$. This is easily seen by noting that $T_e(H)$ is the sum of a maximal toral subalgebra together with the root spaces. Note, however, that this sum need not be direct. For instance, think of $\mathrm{GL}_p(K)$ in characteristic $p$. Although it is not direct it implies the surjectivity of our map $\gamma$ because $\sigma$ maps the derived subgroup $G_{\mathrm{der}} \leqslant G$ isomorphically onto $H_{\mathrm{der}}$ and we similarly have

$$T_e(G \times_{Z(G)} Z(H)) = T_e(G_{\mathrm{der}} \times_{Z(G)} 1) + T_e(1 \times_{Z(G)} Z(H)).$$

This is really highlighting that the problems essentially arise at the derived subgroup.

As you observed, with this observation the exercise becomes fairly easy. Assume $\sigma : G \to G’$ and $\tau : G \to G’’$ are closed embeddings then we denote by $T$ the torus $Z(G’) \times Z(G’’)$. This inherits a natural Frobenius endomorphism from $G’$ and $G’’$ and we have a closed embedding $Z(G) \hookrightarrow T$ given by $z \mapsto (\sigma(z),\tau(z))$ which is defined over $\mathbb{F}_q$, so we can form the group $G’’’ = G \times_{Z(G)} T$. We have regular embeddings $G’ \to G \times_{Z(G)} Z(G’) \to G \times_{Z(G)} T$ and $G’’ \to G \times_{Z(G)} Z(G’’) \to G \times_{Z(G)} T$ where the first maps are the isomorphisms constructed above. It is then easy to see that we may take your group $G’’’$ to be the group $G \times_{Z(G)} T$.