I liked Judith but think once Edward was gone and having no other kids, Donald could return as principal and her character no longer had any purpose. If Donald was no longer around, then I could see Judith staying longer as she would have more purpose with being the school principal.
If Judith had more kids, then definitely, she would have more reason to keep her around.
I think the main reason why Willow turned on Colby was because she realised he was going down either way. If things kept going Colby was not only going to bring Dean down with him but possibly herself and maybe even Bella and Chelsea.
I do think Willow didn’t want to stay living in Summer Bay after everything that happened and she only wanted to come back to try and get back on an even level with Dean and Bella. While she did get them to understand why she did what she had to do but I do think deep down Willow knew things would be never quite same again and so she thought it best to distance herself.