Noidy,
First I'm so glad to "see" you again.
Second and most important - Your father is so wrong about all this IMHO.

Your grandfather loves you. You not being around hurts him much more than the truth. He deserves to see you, love you and know truths. Is he a child or senile?

Kitty is right. Set up seeing him another day, so neither of you has to deal with others.

Of course you love your dad and you should. I'm sorry he is like this on the subject of MS. But that doesn't change reality. Joan Didion (an author, she also has MS) was talking about her DH's death sending her into "magical thinking", a sort of denial of unpleasant real truth.

I too am Shaking my head, but love your recognition that it's "dad's problem".