You asked "Would there be a benefit of defining the Plimsoll boundary explicitly, making clear what are the responsibilities on each side of the line?"
For what it's worth, I think that's exactly what's done in the United States federal government (and presumably mirrored in the various state governments). "Political appointee" in the U.S. is not just an informal description; it refers to a formal classification of employment, with different restrictions, different ethics requirements, and less job security than the non-political employees. Two references: U.S. political appointments and U.S. federal civil service.
While there are occasional abuses and blurrings of lines, as is inevitable, my impression has been that in general this system works surprisingly well. It's rare for a non-political civil servant to be sacked for political reasons -- true, there were some recent high-profile instances of it happening in the U.S., but they were high-profile in part because one doesn't see that very often. And it's even more rare for a political appointee to expect or seek the job security of a non-political employee. Although there are occasional instances of so-called "burrowing in" right before an administration change, most political appointees have better options than that, and accept the precariousness of their position because it comes with subsequent security by other routes anyway.
You asked "Would there be a benefit of defining the Plimsoll boundary explicitly, making clear what are the responsibilities on each side of the line?"
For what it's worth, I think that's exactly what's done in the United States federal government (and presumably mirrored in the various state governments). "Political appointee" in the U.S. is not just an informal description; it refers to a formal classification of employment, with different restrictions, different ethics requirements, and less job security than the non-political employees. Two references: U.S. political appointments and U.S. federal civil service.
While there are occasional abuses and blurrings of lines, as is inevitable, my impression has been that in general this system works surprisingly well. It's rare for a non-political civil servant to be sacked for political reasons -- true, there were some recent high-profile instances of it happening in the U.S., but they were high-profile in part because one doesn't see that very often. And it's even more rare for a political appointee to expect or seek the job security of a non-political employee. Although there are occasional instances of so-called "burrowing in" right before an administration change, most political appointees have better options than that, and accept the precariousness of their position because it comes with subsequent security by other routes anyway.