In life never stoop to anothers lower level period. This is about respect for your father and whatever your beliefs are in a higher being a funeral/memorial/celebration of life whatever you want to call it is about that person not about the rest of the family. It is about showing respect for the person who died. I went to my father's memorial and I am glad I did.
There is also nothing wrong with someone coming up to you to give their condolances either. YES you heard me thats right. They wont get into a long conversation with you but it is perfectly fine to tell the D'ffd person "I am sorry for your loss." Or something similiar. I have seen this done with a friend of mine when his grandma had her memorial at a KH, and also with a relative in a different hall.