I have taken a time out to reflect. Thank you dspreadbury for your observations. They worked. Along with the use of a new layout (thanks Derrek) the solution is potentially labour intensive. I think an engraving reset of any chosen flow would be a very helpful function. Maybe one will appear sometime.
In the meantime I have been forced to a clearer understanding of this complex product. (A good thing.) I now am clearer of the respective scopes of engraving changes, master page assignments, layouts and projects. It took time and the above problem served me well by nudging me into the necessary several hours study and experimentation.
An important conclusion for me was the realisation of the power a hierarchical approach: Firstly take care in designing layouts and then make presentation adjustments to any given flow by experimenting with frame breaks and system breaks (applied in that order); the more fiddly engraving tools be invoked only if the other measures really do not achieve what I need. This hierarchical approach (for me at least) maximises the benefits of the good work of the layout algorithms and maximises reuse while minimising the problems of rewinding and removing engraving edits.