Hello saif
I will attempt to answer all your questions in this one post.
Graceful restart informs OSPF neighbors that the router will reset its OSPF process. The neighbor routers maintain their neighbor adjacencies, but the router performing the graceful restart actually disengages its adjacencies. So from the point of view of the neighbours, the adjacency stays up, but from the point of view of the router performing the graceful restart, the OSPF process is stopped, restarted, and adjacencies are reestablished.
An OSPF router that is gracefully restarting will exit the graceful restart mode only under certain circumstances. These are clearly described. A change in the LSDB during the restart will cause it to exit. Any LSDB change after, will not affect the graceful restart since it will have been completed.
Yes, this is exactly correct.
This command is necessary to be enabled on helper routers when you want the helper to terminate the graceful restart of its neighbor if the helper receives an LSA that will have to be relayed to the restarting router. This will eliminate the need for the helper router to send the LSA to the restarting router and having the restarting router detect this and cancel the graceful restart. It saves the router an extra step.
I hope this has been helpful!
Laz