What should a worshiper do if the imam does not perform some of the established Sunnah acts in prayer, such as raising the hands before and after bowing; should he follow the imam or perform the Sunnah?

The worshiper should perform the recommended Sunnah acts that they deem legislated, even if the Imam does not perform them, as long as it does not lead to disrupting the following of the Imam by being late or early. Raising the hands before and after bowing (rukūʿ) does not disrupt the following, so the worshiper should perform it. As for the rest-sitting (jalsat al-istrāḥah), if the Imam does not observe it, the worshiper should forgo it. Sheikh Ibn ʿUthaymeen said that the worshiper performs what they consider a Sunnah if it does not entail being late or early compared to the Imam, such as raising the hands when saying takbīr for bowing and rising from it.