I'm going to have to go with no. My old guy is 12 and has been raw fed (so no grain; I haven't even done veggies for the last 4-5 years) for about 10 of those years (the later ones!). He still has quite a few lipomas. If the lump doesn't interfer with his movement, I wouldn't get too excited about it.
I don't believe so. I have heard many things about grain free foods helping to starve cancerous lumps/tumors.. but as for fatty tumors, I don't think it really has any effect. Though I don't know for sure!