[Scea91]

While I agree that there are problems with the paper, I think you are confused about suitability of ROC, PR and how test set class imbalance affects them.

Your first two suggestions combined together are very wrong. If you made the test dataset balanced and then measured PR curve the precision would be way too optimistic as it is directly affected by the class imbalance. ROC curve on the other hand is invariant to the test set imbalance.

You can find interesting this short article I have written about this problem: https://arxiv.org/abs/1812.01388