1) Does this mean that the ESB has created two sessions (with the subscriptionID & clientID) but 15 consumers for each session?
I would say yes however the ESB guys would know better, if they do it the same way as MDB's then it will be 15 sessions with one consumer per session.
2) Though my subscriptions seems to be multi-threaded, is there anything else I should be aware of regarding the threading model, or can I continue to naively regard my solution as a multi-threaded setup?
If all the consumers are in the same session then your application wont be multi threaded, howevere like i say, im not an ESB expert and im not sure how this is handled.
Maybe you could give me more info