Yep, season 3b had a winter premier following the summer season (which ended around the same time), but the cast basically had to shoot two seasons in a row, so it makes sense that there's some time between them this time. A lot of time. ;-;
I figured Theo was a chimera, I mean why else was he in with the Dread Doctors? It wouldn't make sense otherwise.
I'm sure Derek will be back next half. If not then he'll most likely be back for season 6 (which has been confirmed!)