Well, Kate leaves LC anyway;
People have been saying this because someone claimed you could see the plane explosion from TBoGT in the background during the final cutscene on happiness island. BUt it turned out that it was just the FarClip value not rendering Alderny. The smoke everyone assumed to be from the plane was actually those smoke stacks in Alderny's industrial zone.
C is the canon ending.
IV had two endings. People consider that Revenge ending is canon but was ever officially confirmed?
And just like with GTA V, IV's trailers depict scenes and dialogue from one specific ending - the Deal ending.
There's also the fact that Packie said he could never leave LC since someone would need to take care of Ma. And a certain someone would have to be alive for him to leave LC - which he is seen doing during TBoGT's credits.
So Packie is left with his ma' whichever ending you chose, so most likely his ma' died between 2008 and 2013 or, less likely, but he just left her as well.
It doesn't really say she's leaving though. She just says she wants to get away from Niko, and is cutting contact off from him. And if she thinks "Packie is on a crash course too" why would she leave Ma? I haven't played through IV in a while, but doesn't she say something similar like Packie, whereas she can't leave Ma?
EDIT: Oh, and Packie leaves just a day or two after IV's ending. TBoGT's "Departure Time" takes place the day after IV's final mission, and during TBoGT's credits Packie is seen getting on a plane and leaving. It's still 2008, it's still Fall, and Packie leaves (most likely for Los Santos).