1) Color correction, especially when done with separate application from the NLE, should be done AFTER picture lock. There are some measures if there are tweaks after, but they aren't 100%. Always wait to color correct until after you have picture lock. But you can render out of Color, media with handles, so you can do minor tweaks. And if you add new shots, make those into separate projects, send them to Color, grade, and send back.
2) You need to either read the manual, or look for a tutorial on Color. It goes through all the steps. Send to Color...grade in Color...render...then SEND TO FCP. That'll send a new sequence to FCP. Not a rendered out QT file...the full sequence.