Of course he can! All cats come again eventually. Keep setting out food he likes and the giant cushion and he might come. Remember to check his top 3 goodies...
Don't worry about the collar. It'll be removed soon enough, as long as you don't stray too far from your objective when first confronted by the Brotherhood.
I don't think it matters whether you fight Bowser before getting all the stars or not. As long as you have 120 stars when you defeat him, you'll get Luigi.
No, not quite. The fox will still wander around, except it won't run away. It'll walk off, essentially. It could stay by you if it wants to, but I keep...