I agree with you both.
Note that only one Venn diagram is acceptable - that between Arthur and Nimue.
By contrast the Mordred Cabal is like Saruman in LotR once he has gone over to Evil - he will not change as he only wants power for himself ... and chops down trees for fun.
I'm jolly happy how ping is playing Walker (I just wanted a reliable fighter-type who had good knowledge of Sherwoodshire and was chatty

)
Clearly Walker wears a symbol of Arthur as he was brought up in the Arthurian Keep (also it helps folk to trust him), but he has sympathy for Nimue (and shares her beliefs.)
Choie is right about the alignment system.
It's pretty basic for these reasons:
- it's easier for me to run
- it makes sense given a feudal system and Beings who take a close interest in the world
I'm happy for there to be some overlap between Good + Neutral and also enjoy seeing Evil being converted to Neutral. (N.B. This is not frequent e.g. Evil Clerics will never turn.)
All intelligent inhabitants will take a belief in Arthur, Nimue or Mordred.
Walker has spent some time with one Lizardman community and can tell you they were Nimue (makes sense since they rely on fishing.)