Darian said that his work with Disney (writing songs for an animated movie) couldn't allow him to do the shows, but if I'm correct he'll be back for the September shows at the RFH. Don't know about Scott though...
Scott was at one of the West coast shows not long ago (as was Darian). I don't know what it is, but he (Scott) had some commitment he had to keep so he couldn't go to Europe. IIRC, both will be there in September from what I read elsewhere.
Neither has left the band or anything like that. That's good, because with no offense meant to Billy or Gary, Scott and Darian are much better musicians - and Scott is certainly much more versatile than almost anybody in the band except maybe Probyn.