1. Can any other tenant object or file any case to stop the sale of property to me by landlord ??
If there is no agreement of sale pending between them, they cannot object to this without any valid reason.
2. As the landlord permits me to start occupying other vacant areas , can I start taking possession of vacant premises and roof ?
If the sale agreement permits you to do so, there cannot be any legal infirmity in it.
On what other grounds a tenant can file case or ask for stay order against the prospective landlord ?
No reply can be rendered by predicting his intention
On what grounds , if at all, other tenants can file a case in court for any stay order against the prospective buyer / landlord ???
Their intentions cannot be predicted.
Can one register a property in his favour with schedule of payments going beyond the property registering date ?.
If the vendor agrees to it, then it can be possible.