Start from the homomorphism $\theta(\epsilon/2)_{W_{\epsilon/2}}$ constructed in Proposition 4.4. The boundedness (continuity) of this homomorphism of Banach $k$-algebras implies that there exists $C_{\epsilon}>0$ such that

$$\forall\underline{t}\in V_{{\scriptscriptstyle\bullet}}(L_{X|Y}),\quad\vvvert\underline{t}\vvvert_{\phi,X|Y}\leq C_{\epsilon}\cdot\vvvert\underline{t}\vvvert_{W_{\epsilon/2}}=C_{\epsilon}\cdot\sup_{z\in W_{\epsilon/2}\cdot}\lvert\underline{t}\rvert_{z}.$$

By Proposition 3.26, one has a canonical homeomorphism

$$\mathfrak{M}(\widehat{V}_{{\scriptscriptstyle\bullet}}(L_{X|Y},\phi(\epsilon/2)_{X|Y}))\simeq\mathfrak{M}(\widehat{V}_{{\scriptscriptstyle\bullet}}(L_{X|Y},\phi|_{Y}(\epsilon/2)))$$

from which one deduces

$$W_{\epsilon/2}\subseteq\mathfrak{M}(\widehat{V}_{{\scriptscriptstyle\bullet}}(L_{X|Y},\phi|_{Y}(\epsilon/2))).$$

Remember that since $\vvvert\mathord{\cdot}\vvvert_{\phi|_{Y}(\epsilon/2)}$ is power-mutliplicative, by Theorem 2.30, it is the supremum norm on $\mathfrak{M}(\widehat{V}_{{\scriptscriptstyle\bullet}}(L_{X|Y},\phi|_{Y}(\epsilon/2)))$. Hence by comparing supremum norms on these two closed sets, we get

$$\forall\underline{t}\in V_{{\scriptscriptstyle\bullet}}(L_{X|Y}),\quad\vvvert\underline{t}\vvvert_{W_{\epsilon/2}}\leq\vvvert\underline{t}\vvvert_{\phi|_{Y}(\epsilon/2)},$$

therefore for any $t_{n}\in V_{n}(L_{X|Y})$, one has

$$\begin{split}\lVert t_{n}\rVert_{n\phi,X|Y}&=\vvvert t_{n}\vvvert_{\phi,X|Y}\\ &\leq C_{\epsilon}\cdot\vvvert t_{n}\vvvert_{W_{\epsilon/2}}\\ &\leq C_{\epsilon}\cdot\vvvert t_{n}\vvvert_{\phi|_{Y}(\epsilon/2)}=C_{\epsilon}\cdot\mathrm{e}^{n\epsilon/2}\cdot\lVert t_{n}\rVert_{n\phi|_{Y}}.\end{split}$$

Let $n_{Y}$ be the integer $\max\{\lceil\log(C_{\epsilon})/(\epsilon/2)\rceil,M\}$, then for any $n\geq n_{Y}$ and any $t_{n}\in V_{n}(L|_{Y})$, there exists $s_{n}\in V_{n}(L)$ such that

$$\lVert s_{n}\rVert_{n\phi}\leq\mathrm{e}^{n\epsilon}\cdot\lVert t_{n}\rVert_{n\phi|_{Y}}.$$

∎