The 787 MEL-LAX isn't an evening departure. It is mid afternoon (~2pm) ex MEL. ~10am arrival in LAX, ~11pm LAX departure and ~10am MEL arrival.
The PER-LHR flight could need 2 or 3 aircraft, depending on the flight times and how long it sits at LHR. Until more details about that flight are released, all that is know is that it needs to factor in the times for 95/96, the slots QF owns at LHR and the ~23 hour MEL-PER-LHR travel time.
The original point still remains, the first 4 787s will only allow 747s to be retired, if the A380 is QF9/10 MEL-DXB-LHR is removed and those 2 aircraft used on existing 747 routes, presumably MEL/SYD-HKG.