If the boy and girl are kufu (of match) and there is no reason of hurmat (impermissibility) between them i.e. razaat (fosterage relation) etc and nikah is done in the presence of legal witnesses, in this case nikah shall take place and will be valid. However a nikah done without the permission and consent of parents invites misfortune and impermanence; hence doing it without their permission is inappropriate and a cause of regret.
(2) 786 is the number of Bismillah. It is allowed to be written at places where the fear of disgrace is involved.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband