Time travel
We are seeing a version of Kemmler in the main story sequence just after a Fist Full of Warlocks, but well before 1961 on his personal time-line AND before the Word? (Not read Warlocks)
A body swap unless to another strong wizard would leave Kemmler like Luccio, diminshed, but instead summon a version of yourself from another universe kill them (or bodyswap and kill) to leave a perfect corpse behind and time jump so there is no version of yourself to be tracked down in that time period by anyone or anything. You can find that which currently doesn't exis, even in a parallel world. That would explain all of Kemmlers deaths, and Kemmler was merely using the Word as misdirection. If Kemmler is going to break one law of Wizardry, he probably broke them all.
If Warlocks is a clue to Kemmlers modus operandi, so probably is Mirror, Mirror, Mirror Dresden has merely hit upon part of the same idea.
Harry breaks all the laws eventually, perhaps chasing Kemmler/Cowl is how he timetravels