I don't think Dorian is afraid of growing old but he is astounded by how beautiful people perceive him as. At this moment he realizes that people bow to him because of the immense impact of his beauty
I think Basil is hesitant to expose Dorian to Lord Henry because he knows the influence he will instill in Dorian and because he wants to be the only one who creates art of Dorian because he is such a rarity.
Sam- I think Basil is better for Dorian because he sees him as more than an experiment. He may see him as a work of art but at least he doesn't influence his life choices. He wants Dorian to stay beautiful for the art but I think Basil "knows" he won't be that way forever so he treats him like a decent human being.
KFW- I don't think Basil has a large ego at all. He is a true artist and I think he truly believes he was put on Earth soley to create art. We can tell he doesn't have a large ego because he submits to Henry and he could show his beautiful painting to the world but he doesn't.
I think Oliver Wilde's relationship with his father in law somewhat effects the way the men in the story act but I don't think it has anything to do with the men's relationships in the story. It may have something to do with the possessiveness Basil has over Dorian though.