When do "immortal" Devil Fruits respawn?
No, these fruits cannot create an immortal army, they are not designed with that in mind. Even after giving someone eternal youth, that is all he will get, he will just remain "young" forever. Anyone will still be able to kill the user in a fight, he may die to an accident etc. The same applies Yomi Yomi no Mi, you get another life but you are not immortal. Also, even if someone has the Yomi Yomi no mi, and his head gets cut there is no body to return, because he will die again immeditally. 

Ope Ope no Mi respawns after the users death (the one who eat the fruit) because the person who gained eternal life has no "connection" with the fruit itself.