Decrypting without using the private key
If $(G,q)$ are public authentic parameters and Alice publishes $(h,R,S)$, then if Alice later publishes $r$, Bob needs to check if $g^r=R$, which fixes $r$. Consequently, when computing $\log_g S/h^r$ also fixes $h$ and the exponent of $S$ is fixed. Changing $m$ to $m'$ would require to solve $m+rx\equiv m'+r'x \pmod{q}$ for $r'$. However, since $r$ is fixed in $R$, this does not work out without changing $R$ to $R'\neq R$ and same argument for $x$ which would change $h$ to $h'\neq h$ (but this can be detected).

But why should Alice not give $(r,m)$ to Bob and Bob runs $Enc(m,h,r)$ again and checking whether the computed result matches $(R,S)$ previously published by Alice? This would also be more efficient as you avoid computing $\log_g S/h^r$.