I don't have the answer, but I am at exactly the same stage as you. The element has blown (an all too common Neff problem) and would be easy to replace were it not for the fact that one of the fixing screws is unhelpfully obscured by the fan. I have removed the locking nut and washer, but like you, I have come up against a fan impeller that does want to move. I am reluctant to use brute force, but find it hard to see what other option there is. I assume that the fan is reverse threaded on the D section spindle, but I am nervous about forcing it and find it hard to see how I can stop the spindle rotating while I try to force the impellor.
Wish I could be of more help.
Stephen Minter
Stephen Minter, March 2007