Sadly, the best way to deal with this kind of situation is to get a senior government official to make a phone call. That will get you the PCC in one day (heck in three hours, as happened to a friend of mine).
If you don't have any relatives/friends with that kind of clout, then you have to keep cajoling/bothering them till they do it.
Also, I know it's too late now, but you didn't need to decline the ITA. You could have accepted it with proof that you have applied for your PCC, and added the PCC when you got it. IRCC understands that there may be occasional delays and will wait for the PCC (within reason, I don't think they will wait six months, but a few weeks isn't a big deal).