In any event, Frank said in interviews that the deciding factor above all was distance. He and Mirai did not have any problems between themselves. He said IIRC that Mirai would arrive 'exhausted' after a 2 hour drive (4 hours roundtrip). That would tax anyone day in and day out. Mirai's family finances were also sited as an issue. Remember that she lost funding as she went down in the ranks. I suspect this is the reason why her choreography hasn't been top drawer these past few years.
But logistics played the major part in ending their professional relationship.
When Carroll stopped coaching at the Toyota Sports Center near Los Angeles airport and moved to a rink in Cathedral City so he could be closer to his home in Palm Springs, it left Nagasu (and her mother, who drove) with a four-hour commute two days a week. She worked with a variety of other coaches (and trained some days on her own) at rinks closer to her home in Arcadia the other days.
Nagasu told me earlier this year that her mother would not let her live on her own near Cathedral City and that such an arrangement would have taxed the family's finances in any case.
Frank Carroll in the same article quote:
"There was no problem between us as people," he said. "The two-hour drive each way was too much. She was exhausted by the time she got here."
at this point and time it is time to move on. Mirai has 4ccs and the olympic team needs to focus or the US could find themselves slipping down the ice and hitting a wall. Let the kids sleep and let politics and all be rid of.