Is it valid to pray in a fully constructed mosque where Friday prayers are not held due to administrative issues preventing its handover to the Awqaf (Endowments Ministry), knowing that a large number of worshippers in it reduces the chance of its handover?

There is no harm in praying in this mosque, but the neighborhood mosque is more preferable if one's heart is present there. Otherwise, another mosque where one can attain khushūʿ (devotional humility) is more preferable. The officials responsible for the mosque should be advised. It is not permissible to hold Jumu'ah prayer in it if other mosques are not overcrowded with worshippers, due to the multiplication of Jumu'ah prayers without need.