This was covered in some detail in the Insights extended ed sessions. What I’m going to describe below probably doesn’t answer your question, but it hopefully explains the behaviour.
There is a need for the tickbox in Customisation Properties, and it does work in some circumstances. The thing to understand is that anything with an EpiBinding will behave according to the EpiDataView - as you’ve observed, set the EpiDataView to ReadOnly and all controls that reference the field will inherit that property. This is by design. To “break” the link, remove the EpiBinding. Then the field will behave as per properties sheet. Any custom controls that you’ve added which aren’t databound will be configured using the property sheet.