Question about 240V / 5VDC PSU wiring
In countries such as UK and Ireland where polarised plugs are used it is possible to (pretty much) guarantee live (L) and neutral (N) connection in the correct orientation. In the rest of Europe and many parts of the world two-pin plugs are used and can be reversed. Your PS1 also has a reversible mains connector so polartity can not be guaranteed.


  1) Is there an issue with the AC+/- inputs on the PSU and the fact that this will change depending on how someone inserts the power cord in the rear 2 pin socket?


No.


  2) when the Pi is powered down and the unit is not in use, Should I be worried about heat or general waste of electricity in the PSU whilst it's sitting there connected to the mains?


It should be fine. Obviously it will waste a couple of watts in standby and this can be eliminated by switching off at the mains.

Make sure you have a suitable fuse in the line to the PSU L terminal if there isn't one built into the PSU.