Using the option for Cast Hidden Shadow will allow you to cast a shadow from a hidden object. So with that you can set in the Details Panel for the actor Visibile to False. This is for the Editor view, so it’s really up to you how you would like this to be set. For the second part you’ll set the Actor Hidden in Game to True. This means that even if you can see it in the editor it shouldn’t be visible in the game. If you’ve got the option for Cast Hidden Shadow enabled you’ll see only the shadow rendered.
I hope this helps.