Forum:Zombie World + Spirit Reaper + Soul Exchange

I have a face-up Zombie World and Spirit Reaper on my side of the field.

My opponent has a non-zombie deck so I know he doesn't have any zombies in his hand to tribute summon. But regardless of what he has in his hand to tribute summon I have a question. He wants to activate Soul Exchange on my Spirit Reaper to destroy it with Spirit Reaper's own effect and he doesn't plan on summoning anything because he has no zombies to summon. Now can he activate the card if he cannot successfully tribute something?

Also can he activate Soul Exchange in Main Phase 2 after he has attacked for the turn? Since Soul Exchange would normally not let him attack what happens if he already did?

And what about Light And Darkness Dragon...Can you activate Soul Exchange for the sake of activating it if it is the only card in your hand just to lower his attack because you know it will be negated?

And what if the above situation is used for lowering his attack but you have a monster in your hand you could have summoned if the card would not have been negated?