They may be creaming personnel off the Song groups. One noted captain of a Song was transferred to captain one of the nuclear subs, although the type of the nuclear sub is not described.
For training, its not hard to imagine that the two retired Hans maybe used as some sort of land based training ship and simulator.
On the picture, I should note that the two 90 meter subs, which should be the upgraded Hans, appear to sport a darker color than the other subs. In my opinion, the 093s aren't in sea trials, they have been out already for quite some time, and compared to the upgraded Hans which got a new tile coating, the ones on the 093s maybe greying.
As for the 093s being in trials, in my view, given the relationship between the 093 and the 094, the latter appearing like a stretched version of the former, the 094 won't have been approved for production and be on the water unless the 093 would have gone into trials first and get certified.
If I were to speculate on the numbers of 093s based on deployment, that would be 3 here at least on Jianggezhuang, one in Xiaopingdao, and at least one in Sanya. I suspect 094s are not stationed in Jianggezhuang. As for the 091Gs, that would be two in Jianggezhuang and one in Sanya, none in Xiaopingdao. The 093 that doesn't have the planes in the sail has yet to be reaccounted, and I suspect that its this design, which I may tag as "093G" for now, is the one on trials.