Shorter proof of $R/I$ is a field if and only if $I$ is maximal

Both directions of your proof are wrong. If $x$ is not a unit in $R$, it's still possible for $x+I$ to be a unit in $R/I$. If $x$ is not in $I$ and not a unit, it's possible for $I+(x)$ to be $R$. In both cases, you can take $R=\mathbb{Z}$, $I=2\mathbb{Z}$, and $x=3$.

I think m.k.'s comment is right on the money: assuming you can prove that a commutative unitary ring is a field iff it has no non-trivial ideals (when by "trivial ideal" here we understand the whole ring and the zero ideal.):

$R/I\,$ is a field $\,\Longleftrightarrow \nexists\,\,\text{non-trivial}\,\, J/I\leq R/I\Longleftrightarrow \nexists\,\,\text{non-trivial}\,\,I\lneq J\lneq R\Longleftrightarrow I\, $ is a maximal ideal.

$\mathfrak m$ is a maximal ideal $\Leftrightarrow R/\mathfrak m$ is nonzero and has no proper nonzero ideal.

$\Rightarrow$: Suppose $\{0\}\ne I\subset R/\mathfrak m$, $I=\{a+\mathfrak m:a\in A\}$, and $A$ is as large as possile. $I$ is an ideal, so $a_1,a_2\in A\Rightarrow a_1+\mathfrak m,a_2+\mathfrak m\in I\Rightarrow(a_1+\mathfrak m)+(a_2+\mathfrak m)=(a_1+a_2)+\mathfrak m\in I\Rightarrow a_1+a_2\in A$

and $a\in A\Rightarrow a+\mathfrak m\in I\Rightarrow(r+\mathfrak m)(a+\mathfrak m)=ra+\mathfrak m\in I\Rightarrow ra\in A$ for any $r\in R$. $A$ is an ideal. $I\subset R/\mathfrak m\Rightarrow A\subset R$; $A$ is the largest $\Rightarrow\mathfrak m\subseteq A$; $I\ne\{0\}\Rightarrow A-\mathfrak m\ne\emptyset$.

$\Leftarrow$: $\mathfrak m\subseteq\mathfrak n\subset R\Rightarrow R/\mathfrak n\subseteq R/\mathfrak m\Rightarrow R/\mathfrak n=R/\mathfrak m\Rightarrow\mathfrak m=\mathfrak n$.

$R$ has no proper nonzero ideal $\Leftrightarrow R$ is a field.

$I\ne\{0\}$ and $I\subset R\Rightarrow$ no element in $I$ has an inverse $\Rightarrow R$ is not a field;

$R$ is not a field $\Rightarrow\exists$ nonzero $r\in R$ such that $1\notin rR\Rightarrow rR$ is a proper nonzero ideal.