Create a new layer on top and put the page numbers on it.
Work with more layers would be very useful, I recommend to work with following layers:
Background—Image—Text—PageNumbers (some more too).
Does not cost space, but helps to work correctly.
I would also strongly recommend not to put the page number inside the page margins, these are for text, not for stationaries like page numbers or running headers. All textframes should align with the margin guides, but in your case would be a conflict with the number.
In order to make some page numbers white, you can duplicate your master page containing the page numbers and name it something like "white page numbers", apply this master to the desired pages. Another way is to overide the master page items, allowing you to change the desired page number to white, the disadvantage is that you might forget which pages you changed and subsequent changes to your book might result in a white page number on a white page.
Either way you’re going to wind up with that problem.
There’s no substitute for proof reading.
Agreed. A master page with an unusual name will appear in the pages panel, helping to highlight any issues.