I have an older Oberdorfer on my 5411. Can't figure out what that circlip is supposed to do. The impeller is held on by the pump cover and the shaft can't go any deeper into the housing. It does nothing. It's still there on mine but if I ever lose it I don't think I'll replace it.
If the impeller sticks to the shaft, the shaft will pull out. No problem, just slide it back in. It's actually a lot easier to get the impeller and circlip back on with the shaft out. Just rotate the shaft as you push it back in. At first it won't go all the way in, then the slots will align and it will go the rest of the way.
The older pumps have an adapter between the pump shaft and cam shaft than can require removing the whole pump to get it back in right. I think yours is new enough not to have that. The difference is the end of the shaft. Of it has a female slot, there is an adapter. If its a male tang, no adapter. Mine has the adapter but I have been able to put it back in without removing the pump.