intercession

What is the viewpoint of Islam regarding one who beseeches before the Prophet's grave or elsewhere "O Allah's Messenger, ask Allah to forgive me" or "O Allah's Messenger, pray for me"? Also, what is the significance of Allah's words (If they had only, when they were unjust to themselves, come unto thee and asked Allah's forgiveness, and the Messenger had asked forgiveness for them, they would have found Allah indeed Oft-Returning, Most Merciful)  (An-Nisaa' 4: 64)?

Dear Scholar,Assalam-u-Alaikum!Is the hadith about a martyr interceding on behalf of his 70 family members considered authentic by the scholars? And will intercession take place only on the day of judgement or it can also take place in the grave? For example, if the parents of a shaheed are getting punished in the grave for their sins, does the shahid have to wait till the day of judgement to intercede for them or he can intercede in the grave so that Allah (SWT) forgives their sins and save them from the punishment? And what reward the parents of a shaheed get if they have not encouraged their son to become a martyr?Jazakahllahu Khairan!