I don't know if you realize this but they tried out Aleksi, ALEX, and pronax. Kicking karrigan at the time was the right move if they had a backup. NiKo is not as toxic and egomaniacal as people make him out to be. There has just been no IGLs on the market worth picking up that fit the style of the team. There were really only 2 people who would have been a good fit for the team. Nexa and blameF, who were sleeper gamble pickups by orgs that worked. So it's not like they were known entities on the market. Also, let me just say this, karrigan is mad overrated. If you look at what he has done with mouz, just like FaZe and TSM, they were good for a short bit of time in which they looked to be the next era-defining team, and then after 6 months, they got fucked. I mean sure you could attribute it to the online era and all that but honestly, no team goes from the level they were at to the level they are at now just because tournaments went online. As for a good move to fix the team, I would just say disband. Cold is actually a really good role player. He is also one of the greatest clutches of all time. So, I actually think the fix for G2 would be to take cold and sub him in for AmaNek and then take NiKo and put him in for JaCks. Then rain can sub in for frozen on mouz and broky can take chrisJ's spot.
As for why FaZe should leave CS, I think it has been a long time coming. They have not had success despite massive investments and CSGO is not as profitable for orgs as it used to be.
TLDR: The FaZe situation isn't entirely NiKo's fault.