Final answer:
Pam's grandmother's name in S6E4 "Niagara" is unknown.
Step-by-step explanation:
In the TV show "The Office", specifically in episode S6E4 "Niagara", Pam's grandmother's name is not mentioned. The scene you provided from another source does not contain the answer to this question either. Therefore, the name of Pam's grandmother who almost doesn't come to the wedding is unknown.