Jobs...
From: someone
Dancer
Do dancers do other things besides dance?
Do they function more like hosts?
How often do dancers double up as escorts/whores?
Dancers who actually expect to receive any tips do function somewhat as hosts. That is, they talk to the guests as well as dance with them, making the experience of the visitor more interesting and fun. Some also do sex work; some don't.
From: someone
DJ
Do DJ's double up as dancers, since they can DJ while leaving their avatars dancing?
Some DJs dance while in-world. Others prefer to sit at the control table. In either case, their main job is song selection, and offering the service of streaming music to SL (a service that carries a non-trivial RL cost).
From: someone
Model
Why do clothing designers need models?
If multiple avatars are needed, Is it really that hard to overlay multiple avatars of yourself in an image editor?
If you're talking about just doing poses for promotional shots (putting on the vendor pictures or whatever), there clearly isn't any real
need for a model; the designer could just wear different appearances and take pictures of him or herself. Still, having models is fun. It may also help to generate additional buzz for your designs, as the models will likely talk about what they did, proudly wear the designs they modeled, and so forth.
If you're doing a fashion show event, having models is necessary. One person simply wouldn't be able to handle all the quick changing required.
From: someone
Escorts / Whores
Are these the highest paying jobs without being a content vendor?
Do escorts normally work 'on site' at the clients place, or at their own premises?
Quite likely. Certainly, their typical hourly rate is higher than the other jobs you've talked about. Given the abundance of willing amateurs in SL, the popularity of escorts is a bit puzzling to me. But people may enjoy role-playing paid sex, or they might want to try unusual sexual acts that would be difficult to find somebody to play with them. Or maybe the better escorts are just really really good at what they do.
As for where they work, I've seen both of those, as well as escorts that mostly work at clubs, using special rooms provided by the club. All a matter of negotiation between client and worker.
The only other type of job that I didn't see on your list is instructor, which is on the borderline between content creator and not. Certainly, teaching a class that involves building of any sort requires skill as a creator, even if the instructor isn't actually building anything him or herself in the class. Classes like the one NCI offers on buying land don't involve creation skills, but teaching one requires substantial knowledge of SL.