Delivery constraints? Not sure what you mean by that, there's 8 more to coe by January.
QF partnership does not mean that EK will reduce flights, only that EK can sell tickets on QF and vice versa. It's a standard condeshare agreement. The only thing changing is, as you said, the routing of the QF Australia-LHR sectors going via DXB instead of SIN.
Could be that they have a lot of Aussies on the books already, EK like to even out the nationalities so if there are getting to be lots of one group they will hire into the other groups until the numbers equalise. Very normal for them and has been going on for years. They want to keep a good mix of crew from all over, not to be too dominated by one or the other nationality.