Doubt on PSRR calculation and result
Question 1: 

Yes the method sounds correct. But, you do not need to add a new VDC source in series with the fixed one. Just set AC magnitude of your 1.8V DC source to 1. 

Question 2:

The PSRR+ of such opamps will be bad because looking from the supply to the output, it looks like a common base amplifier! Improving PSRR is a very complicated but interesting topic, but most of it boils down to trading off headroom for better PSRR. For a start, look into other opamp topologies. Generally speaking, cascoded outputs offer better PSRR etc. If you have headroom, you can basically precede your opamp with a NMOS regulator. This will essentially cascode your whole opamp  from supply noise. Ofcourse, headroom might be an issue now.

Another more sophisticated way to improve it for your opamp, is to some how move the gate of PM67(and/or any other transistor that looks like a common base amplifier from the supply) in sync with the noise at its Source, this way: $$\Delta Vgs = 0$$ To do this you can try a small capacitor between source and gate or even create a secondary gain loop that injects correlated supply noise at the gate of PM67. Yet another way is to drive the body of PM67 or NM28 in a way that cancels supply noise