You can either remove the spear and rack into the keg, or remove the gas side check valve from your coupler and fill it through the coupler with the spear in place (or hold the pressure relief valve open instead of removing the check valve).
Getting the spiral retaining ring back in is way easier than taking it out. Just get it started by hand, and when it gets to where it has to overlap itself squeeze it in with pliers.
If you orient the ring so that the end on top is near one of the notches, it will make it easier to remove next time.