All praise be to Allah, peace and blessings be upon Prophet Mohammad and upon his family and righteous companions.
In principle, the Jumu’ah (Friday prayer) is to be held in the biggest mosque of the town, but if it is small for the praying people, then in this case, it is permissible to organize a second Jumu’ah in other mosques and as much as needed. However, if the biggest mosque takes all the praying people, then other mosques are to be closed during the Jumu`ah prayer.
Ibn Hajar Al-Haitami (May Allah have mercy on his soul) said: “ The third condition for the validity of the Jumu`ah is: It shouldn`t be preceded by another one, or coincide with it in the same town because it didn`t at the time of the Prophet and the rightly guided Caliphs except in one mosque. The objective here is for the praying people to gather in one place except in case their number grew bigger…..and it was difficult for them to gather in one mosque.”{ Tohfat Al-Mohtajj 2/425}.
Accordingly, the above Musalah should be closed and the eighty praying people should head to nearby mosques as they are big enough to take them as well as others.
As regards using pulpits for delivering sermons (Khutab), it is a Sunnah. Ibn Hajar Al-Haitami (May Allah have mercy on his soul) said: “ Delivering the Khutbah from a pulpit is a Sunnah.”{Tohfat Al-Mohtajj(2/425)}. If there wasn`t a pulpit , then it is permissible for the Imam to deliver it while sitting on the floor and the Jumu`ah is considered valid since the pulpit isn`t among the conditions for the validity of the Khutbah. However, it is recommended that the Imam leans his back against something since the Prophet (PBUH) used to do so before making the pulpit. Ibn Umar narrated: “The Prophet used to deliver his sermons while standing beside a trunk of a date palm. When he had the pulpit made, he used it instead. The trunk started crying and the Prophet went to it, rubbing his hand over it ( to stop its crying).” {Bukhari}. And Allah knows best