Regardless of who is right and who is wrong in the Z vs JCF little feud, you can't really give much credit to JCF for the title in Washington when he was barely part of Z's team at that point, wasn't there in person (if I recall correctly), and just advised via phone calls. As you have said, there’s only so much blame you can put on a coach, and this also applies to the positive impact he may have had in such a short period of time. I do give him credit for Montreal, though.
The truth is Zverev was having not brilliant, but at least pretty good results the first half of 2017 (wins in Montpellier, Munich, Rome, finals in Halle), then JCF arrived, Z peaked and then completely crashed. Apart from the initial weeks, result-wise, it hasn't been the most productive partnership. It makes sense they, or at least Z, wanted to part ways. This doesn't necessarily mean JCF is a bad coach and/or Zverev is a bad person/athlete, both of them have proved they know and respect the sport, and have what it takes to win titles (had in the case of JCF, and Z to a lesser level, but still).
Other players have split with their coaches with the same or even worse kind of comments being exchanged, yet people in this forum have barely given them attention or tried to find who is the good and the bad guy in the situation. When Z is involved though, most users are eager to crucify him
even though everything points out he isn't the only one to blame for the failed partnership.