Spiney Norman
You missed out one key element of the training scheme, the OTU (Operational Training Unit).
Here, after individual training, crews would come together, less the Air Engineer as you mention, and convert to the Wellington. They would learn to fly and operate as a crew right the way up to actual ops that would always involve leaflet raids and sometimes actual ops such as the first 1000 a/c raid on Cologne.It was quite a comprehensive but intense course.
The crews would then convert to relevant 4 engined heavy at a Heavy Conversion Unit, of which Riccall was one. So these guys weren't as green as you suggest and they would certainly have more than 150 hours. The loss rates at these OTU and HCU's by the way was very heavy and many instructors were killed while serving on them in between operational tours.
Suit