Is it ok for an examiner to hide exam reviews
My personal view is that you should be able to get feedback on your exam, though that doesn't necessarily imply getting a marked-up copy of it. But that is a personal view and whether there are rules or not at your university, the personal views of your professors hold more weight than mine. 

It may be that there is some fear that giving you this material will make future use of the questions impossible - security through obscurity.  

But what is OK in one place is forbidden in others. 

But you are more likely to be successful if you ask for feedback on your performance. It is hard to argue that you are not entitled to that. 

I doubt that there is fear that you have been too harshly judged in the grading, but there may be a reluctance to do anything that might generate such a charge. 

But, as you are aware, without feedback you have no way forward other than blindly guessing. 

One possibility is that your grades are being reviewed by a higher authority and so there is a delay. If making a bit of noise has been effective in the past, you might be able to successfully call for a higher level review without seeming like malcontents.