Can "Daj" refer to someone mentioned previously? For example: "val qeylIS. jaghDaj (i.e. Kahless's enemy) HoH Suvwl'"?
Yes, that's right. Much like "his" or "her" in English, -Daj is a very general reference and might refer to any of a large number of people. In both the English, "The warrior killed her enemy," and the Klingon, jaghDaj HoH SuvwI' it is not specified which "her" we are referring to. It might be the warrior or it might be some other "her". Without context, we don't have another "her" or "him" for it to be referring to, so I would assume that we are talking about the warrior's enemy. But in your example, you are giving a little more context and I would be more likely to assume we are talking about Kahless' enemy in a case like that. If you really want to make it clear, you can be specific about who's enemy they are: qeylIs jagh HoH SuvwI' "The warrior killed Kahless' enemy."
If by that you mean the placement of the subjects and objects is the reverse from where they are in English, then yes.
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