Is the congregational prayer which is led by an Imam who cannot sit between the two Rak’as valid?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.

Referring to your question, sitting between the two Rak’as is an act of Sunnah, but can he sit between the two prostrations as the latter is a pillar of the prayer? If he prays alone, the prayer is valid’ if he leads a prayer as an Imam, he must perform all pillars of the prayer as much as possible.