I'm glad others have spotted ambiguities. When I checked the other site yesterday yesterday nobody made any reference to ambiguities, though one such comment appeared later.
There are actually four possible ways of getting the main subject, and s_pugh is right to identify two ways of getting the shortest one.
Like others, I think it's probably safe to conjecture that the intended solution consists of highlighting all the subjects in straight lines, but I dislike the ambiguity, which could have been avoided by a suitable addition to the preamble. Even so, I am not 100% convinced that my proposed highlighting is correct, First it involves two items sharing the same letter, which is not ideal,; secondly it is impossible to enter the last two letters of the main subject so that both that word and the relevant crossing entry can be read correctly, despite some suggestions below.
I'm astonished that the editors didn't spot the ambiguities and amend the preamble or even the grid. Or perhaps they did spot them and expect us to be mind readers.