I may add the detailed argument later, but first the ball by ball;
I eventually agreed to confine my inquiries to 'Departmental policy on" my issues and "the material the department had available to in in reaching such policies" - that is, the upper level papers and the files they drew on. This may well get me all the papers I need (though it's still open to the Department to say yes they have them but they're exempt) and certainly will be much faster than waiting for a hearing, so in practical terms it's more or less satisfactory, but it suffers from the general failing of civil law; it's centred on settling the individual dispute rather than on the interests of society at large, which in this case are best met by having the Tribunal beat the FOI section around the head and shoulders with a silver-headed cane until they agree to change their utterly improper policies and their fundamental attitudes to transparency - the latter well illustrated by a straightforwardly mulish refusal to admit at any point that the Department held any files that weren't individual client files (policy files, for example, or issue-based files).