Good grief. We've gone from a system that relied on "things everyone knows" (that, well, not everyone knew), to a system of unclear drafting followed by back channel--but contradictory--advice from the IFAB oracle.
As to why the IFK without a caution or send off doesn't work as the Laws are actually drafted, it's really simple. The Laws say that verbal offenses result in IFKs. That sentence doesn't create offenses, it informs that they result in an IFK not a DFK. The only verbal offenses contained in Law 12 are contained in the lists of cautions (dissent, USB) and send offs (OFFINABUS). There simply are not any other verbal offenses. Saying that verbal offenses result in IFKs is not (or perhaps until DE flips again I need to say "should not in any rational world") an invitation for referees to make up their own list of verbal offenses that they are going to choose to sanction. This is simply insane: if the Laws are going to recognize IFK-only verbal offenses, so be it and I would have no objection--but they need to say what the offenses are. This is absolutely insane.