When our Imaam was informed that it was incorrect to read a kitaab immediately after the Isha Fardh Salaat, he switched to reading before the commence

Answer

We presume that by 'before Isha time' you mean before the Isha Fardh, and not when it is still Maghrib time. If the Imaam prohibits musallis from Tahyatul Musjid or from the four raka'ts Sunnatul Ghair Mukkadah of Isha, then undoubtedly such prohibition is tantamount to kufr, for it comes within the scope of the Qur'aanic castigation of those who prevent from Thikrullah. However, we doubt that the Imaam prohibits musallis from performing any acts of ibaadat. But, if he does, then it will be Waajib for the trustees to prevent the Imaam from such haraam action.

Whether the Imaam prevents musallis or not, musallis should ignore him, and continue with their ibaadat irrespective of the Imaam reading the kitaab. The best option is for the Imaam to read the kitaab after musallis have completed their Sunnat and Nafl Salaat. The argument that the kitaab-reading is more important than the Masnoon Tahyatul Wudhu and the Four raka'ts Isha Nafl, is despicable. Denigrating such ibaadat which has been explicitly commanded by Rasulullah (sallallahu alayhi wasallam) for the sake of a self-invented act originated fourteen centuries later, is deviation and haraam. Displacement of Sunnah acts of ibaadat develops into bid'ah.