QIf I summon a Bahamut with this follower's Fanfare ability, and Bahamut's Fanfare ability destroys this follower, do I still have to return Bahamut to my hand at the start of my end phase?

AYes. Once this follower's Fanfare ability is resolved, Bahamut will have "At the start of your end phase, return this card to its owner's hand," which will remain valid even if this follower gets destroyed.

QIf I summon a Lucifer with this follower's Fanfare ability, do I still give my leader 4 defense even though that Lucifer returns to my hand at the start of my end phase?

AYes. When you proceed to your end phase, all "At the start of your end phase" abilities are triggered and become pending. Then you can choose the order in which you resolve the pending abilities.

In this case, regardless of which ability you resolve first, you would give your leader 4 defense.

QWhen playing this follower's Fanfare ability, if I have another copy of this follower in my hand, can I summon it?

AYes. Note that the summoned copy's Fanfare ability will also trigger.