B747-8 is an intercontinental aircraft, which indicates that it is going t be economical on long to very long distances i.e of flight hours 14 or grater. However the flights PIA would most likely fly are not more than 5 hours which does not make it feasible.
If PIA opted for B747-8 to Fly to USA, Canada or Australia then the required passengers are an issue.
Regarding the issues concerning B787's you should search first as those have been addressed. Today the batteries that are being used are those used on B777's. Delivery dates are adjustable on various factors, if you see over the past 3 years there are many cancellations on orders of B787 so slots are available.
If you look at the time when PIA had actually ordered for the B777-300's then these aircraft would have been already in PIA's fleet for the last 4 years if not more. The reasons why PIA requested to change to B787's was it had to fill in the slots of A310-300's as well as to minimise the losses of technical landing on USA flights.
Initially PIA wanted Boeing to change the B777 order to B737's but this was not allowed by Boeing. Had this option been allowed A320's and the wet leased B737-800's would not have been required.