Did Snape have a failsafe method for giving his memory to Harry Potter? What if Harry wasn't around?
Since Harry wanted to find Voldemort to end him, finding Snape wasn't part of his current mission. It was just by chance that when Voldemort needed to interrogate Snape about the Elder Wand's true loyalties, Snape happened to be there. In his dying moments, he didn't even expect Harry to be there. How did Snape plan to show Harry the truth, like Harry being a Horcrux, had Harry not been around?
As it is often said, no plan survives contact with the enemy. Snape did not have a detailed plan, but was (like Harry) making it up as he went along.
Dumbledore told Snape to "do all in your power to protect the students at Hogwarts" (page 547) and (from page 550; all page numbers from Deathly Hallows UK hardback):
'There will come a time when Lord Voldemort will seem to fear for the life of his snake.'
...
'Then I think it will be safe to tell Harry.'
...
`So the boy ... the boy must die?' asked Snape, quite calmly.
`And Voldemort himself must do it, Severus. That is essential.'
Snape's instructions were to convey the information described above, but only when Voldemort started to worry about the snake.
So Snape (who did not know what it meant; Dumbledore never told him about the Horcruxes) did not tell Harry anything before the evening he died. He did of course do all he could to protect the students (by saving them from a worse headmaster).
Then, during the battle, Voldemort summons Snape. On arrival Snape discovers that Nagini is now "suspended in mid-air, twisting gracefully within the enchanted, protected space he had made for her" (page 516). Up to now Snape has had no reason to believe that his life is in imminent danger from Voldemort, who has been totally taken in by Snape.
Now, Snape sees the snake (page 525)
And for a moment Harry saw Snape's profile: his eyes fixed upon the coiling snake in its enchanted cage.
So Snape now knows that he can tell Harry the information that he promised Dumbledore. He immediately tries to do so:
'No my Lord, but I beg you will let me return [to the battle]. Let me find Potter'
Over the next two pages, he asks Voldemort four times for permission to find Harry for Voldemort. (Of course really he wants to tell him the information.)
But Voldemort does not give Snape the chance, and strikes the killing blow. Even then, Voldemort does not realise that Snape was a double agent: "You have been a good and faithful servant, and I regret what must happen".
In summary, Snape had no backup plan. He had no opportunity to do anything between when he discovered the time was right to talk to Harry and when Voldemort killed him. Furthermore, he had no reason to suspect he was going to die at that time, so why would he have made a backup plan? It is hard to see what kind of plan he could have made anyway.
