Proof of some conjectures of Z.-W. Sun on the divisibility of certain double-sums

Z.-W. Sun introduced three kinds of numbers: \begin{align*}S_n=\sum_{k=0}^{n}{n\choose k}^2{2k\choose k}(2k+1),\qquad s_n=\sum_{k=0}^{n}{n\choose k}^2{2k\choose k}\frac{1}{2k-1}, \end{align*} and $S_n^{+}=\sum_{k=0}^{n}{n\choose k}^2{2k\choose k}(2k+1)^2$. In this paper we mainly prove that \begin{align*} 4\sum_{k=0}^{n-1}kS_k\equiv \sum_{k=0}^{n-1}s_k\equiv \sum_{k=0}^{n-1}S_k^{+}\equiv 0\pmod{n^2}\quad\text{for $n\geqslant 1$}, \end{align*} by establishing some binomial coefficient identities, such as \begin{align*} 4\sum_{k=0}^{n-1}kS_k=n^2\sum_{k=0}^{n-1}\frac{1}{k+1}{2k\choose k}(6k{n-1\choose k}^2+{n-1\choose k}{n-1\choose k+1}). \end{align*} This confirms several recent conjectures of Z.-W. Sun.