Do I need to define all forms of cheating in the syllabus?
I think your problem quite literally is that you specify too much. If you provide a long list of things students are not allowed to do, it is natural that students assume that the list is comprehensive (that is, everything that is not on the list has to be legal). If you do what chubakueno proposes and have a single rule Plagiarising == fail, most people would be fully aware what that means.

EDIT: clearly, this does not mean that you should never go into more detail. Of course, if one has unusual or unexpected rules which other comparable lectures in the same university do not have, then of course they need to be explained. However, most of the examples given by Village are IMHO pretty obvious.

Frankly, many of the items on your list cannot reasonably be assumed to be ok, no matter the rules. For instance, did you really have a student tell you with a straight face he thought it was ok if he pretended to be somebody else to write the other guy's exam?