I agree 200% about Dark spirits disguising themselves as Good spirits. MindReality is right that the devil works by mixing truth in with falsehood. Outright lies will never really accepted by people unless it was mixed in with truth.
Regardless, I am very wary of people who claim to channel beings. I am also weary of Abraham-Hicks. Although some of what she is saying may be correct, some things in it I am sure are false.
Ethereal is also correct that naivete is the main problem when it comes to this sort of thing. When people find a teaching which they believe is correct, they leave reason and discrimination outside and completely accept what the other person is saying. But is humility the only answer? Is there anything else that we can do? If the foundation of our thinking and beliefs are incorrect, then whatever stems from that tree will also be diseased. How else can we keep the roots of our souls pure?
I agree with the list you have provided of people/arts which should be avoided. Just because it is possible doesn't mean that it is territory that must be ventured into. By the way on the list in the link you provided, why are some teachers listed as "fallen"?