This question would be better answered by ST support. Still, here are my thoughts...
Indeed it is possible that the WRP on the first pages which is implied by the ROP does not appear in the WRP values. That's in fact what I would expect. But that should be checked with ST support.
When ROP is enabled, the only change you can make to the Flash is Mass Erase. However, you should be able to make your application disable ROP temporarily. (there are the non-volatile Option bytes and the currently active active ones, initialized at reset from the NVM, and which the application can modify easily) Check the bootloader examples from ST for how to do that.
On some devices it might also be needed to execute the erase from the RAM. Again, check the bootloader examples from ST, and the ST forum.