Don't think it really matter about fire spread jasper (doesn't seem big enough for compartmentation) provided that, as PhilB stated, people can turn their backs and make their way to protected route or final exit. 20M to me is OK in an open design alert situation.
I would be more concerned about your comment regarding the enclosed rear courtyard.
Is it enclosed in that there is no way out unless through the restaurant and if so is this the area that the rear exits discharge into?
I had a similar premises, with rear protected escape routes to place of safety, and did not see a problem at all. The front main entrance/exit was at mezzanine level served by an open 1/2 stairway from upper and lower level. They wanted to initially escape through the kitchen on the first floor rear but they were persuaded to provided a protected corridor from public area.
Very nice place and great food as I found out AFTER.