Is raising the hands for the takbir before the Imam's takbir for bowing, and then lowering them, considered a violation, bearing in mind that the congregants do not bow until after the Imam bows?

The basic principle is that the worshipper (ma'mum) follows his Imam in all aspects of his prayer. It is disliked to raise the hands for bowing before actually starting to bow, but the prayer is valid. The proof for this is what Al-Dusuqi Al-Maliki mentioned in his commentary, that raising the hands before or after the takbir (Allahu Akbar) is disliked.