بسم الله الرحمن الرحيم
(1) No, it is not lawful to erect mosque in the graveyard.
(2) It is also not lawful.
(3) In the endowed land of graveyards only Muslims should be buried. The endower has endowed the land only for this purpose. It is not lawful to use it for mosque construction. It is unlawful to support in unlawful job. Such people shall be sinful.
Had you submitted your question after making the survey of the graveyard land with local Ulama then it would have been far better.
Allah knows Best!
Darul Uloom Deoband