If a player encounters the Reaper, and rolls a 5, can he then have the reaper attack a character who is in the Crown of Command?

Nope! Ran into this a lot with Danse Macabre ending, GR is already in Inner Region.

Generally in Talisman, if you're instructed to do something, but you can't, you do nothing instead. If you roll a 5 on the Reaper, but all other characters are in the Inner Region, then you do nothing. It's a broad generalization, and of course there are exceptions that have to be taken on a case-by-case basis.

Regarding the Danse Macabre ending, I believe a character on the Crown of Command can choose to send the Reaper to a character in the Inner Region. The ending card says "any" Region is allowed. Once the Reaper is in the Inner Region, he can move in that Region like a character if someone rolls a "1," but if he moves to the Portal of Power then he cannot cross back unless someone on the Crown activates him and sends him to a character in the Inner Region.

Those aren't printed rules regarding Danse Macabre, but it's how the Digital Edition handles it, and I think it works well. It's quite scary to see the Reaper chase you into the Inner Region where you're normally safe from him.

Reaper, along with the Werewolf are mentioned here:

http://community.fantasyflightgames.com/index.php?/topic/99396-the-werewolf-npc-in-coc/?hl=werewolf