For me, Loew is the best option, but he's not going to give up a crack at defending the World Cup, and the German FA aren't going to let him manage everton in the interim whilst there's a World Cup to win
To me, the best option is to give Unsworth responsibility for the day to day running of the club for the rest of the season and have him report back to Loew, who can manage Germany till after the World Cup but already be preparing things for 18/19 season
Get everything arranged nice and early
Reassure Unsworth that he will be part of things going forward, get a financial package with both Loew and the German association agreed and communicate it all to everyone
Act early and act decisively