For Chris, the obvious choice for him is "That's All Right". He could be really good with that song. He should then take the tempo completely down and close with "Love Me Tender", if for no other reason than to save his voice, but also to show versatility.
This week could be a struggle for Elliott, but there are songs he could pull off that could be gold for him. "Its Now or Never" could be tremendous for him. A little tempo to show some versatility and a song to display his superior range. For a slow song, I'd say go with "Falling in Love with You". Only concern I would have with the song is it is a low octave song, but I think he could get the job done with it. Order of song will be critical for him. I'd open with "Falling in Love with You" and when the rest of the four are slowing things down on their second song, he finishes with "Its Now or Never" as a contrast.
Katherine I think has been dealt a bad hand with this week, which I think is for the best, because she should have been gone for a while now. I'd go with "Are You Lonesome Tonight" and "Burning Love". I don't think she will survive in a week where the songs are from a male artist, particularly one with such a distinctive style that she will not be able to give her own spin to.
In order, I expect them to finish Chris, Elliott, Taylor, Katherine, but it all hinges on song choice, which this bunch has done a really bad job in many cases.
I think Return to Sender is too blah for Taylor. I still like One Night for him because its bluesy and that's more his style. Little Sister would be good, too.