I think there are different philosophies at work here regarding the technical aspects of singing. It seems you have technical requirements that you apply separately from aesthetic requirements. I don't do that. From my point of view, someone absolutely cannot be a great singer and not know how to sing. This is an absurdity--like saying you can be a great football player and not know how to play football, or a great writer and not know how to write. If you are a great singer, then you know how to sing. It cannot be otherwise. This is because, in my philosophy, the technical aspects of singing are there only to serve the singing, the communication of the music and message.
I think my philosophy is based on my being an ethnographer with training in Anthropology, Folklore and Ethnomusicology. One thing I learned was that different traditions have entirely different standards, both technically and otherwise. Louis Armstrong and Luciano Pavoratti have nothing in common technically, but both are great singers. Norma Waterson has nothing in common with either of them technically, but is also a great singer. Kaluli tribespeople in Papua New Guinea have great singers, too...again, with no technique in common with the others.
What is usually happening when people complain that English gypsy singers can't sing (these are Norma's main influences) is that the commentators have been trained in a particular tradition, be it classical, jazz or whatever. In other words, they apply technical standards from completely outside the culture being commented upon, standards that the singers would not even recognize as valid. That's generally the problem with a "technical" approach--how do you even know what technical standards are appropriate to apply? You have to do a lot of studying of English source singers before you can even say if Norma has technical proficiency or not.
Granted, Norma sings songs from outside this tradition, and that may be what bugged people in the first place. It is perfectly valid for you to say that she isn't good at Jazz standards, if that's a tradition you are well-versed in. But to say she flat-out cannot sing, as Challis did, is to presume that you know a lot about her tradition.
Of course, the more important question is: do you like her singing? If not, don't listen and if so, listen. That's the simplest philosophy of all.