"1. Why won't Russia even attempt to destroy a single bridge across the Dnieper?"
You don't destroy what you intend to use. Retreating armies destroy bridges. Advancing armies attempt to preserve them.
Russia's aim thus far has not been to capture territory so much as to degrade Ukraine's military, which is achievable by a strong defensive position, provided your enemy is foolish enough to attack them, which thus far has been the case.
"1. Why won't Russia even attempt to destroy a single bridge across the Dnieper?"
You don't destroy what you intend to use. Retreating armies destroy bridges. Advancing armies attempt to preserve them.
Russia's aim thus far has not been to capture territory so much as to degrade Ukraine's military, which is achievable by a strong defensive position, provided your enemy is foolish enough to attack them, which thus far has been the case.