This question is not useless. It is not stupid. It is certainly not easy.
American English is my native and only language. Even so, I find that I'm not completely sure whether this specific instance of "understanding" is countable. I suspect that it is not, but I can't quite prove that it isn't.
On one level, we're talking about one understanding of Chinese sensibility, in contrast to other, better understandings of it. I have an understanding. You have an understanding. Christine has an understanding that is (or so we can easily assume) worse than both of ours. That's three understandings that I can easily count.
On another level, we're not talking directly about an understanding. We're talking about a lack. I can assume that her lack is more severe than my lack. I can further assume that my lack is still quite significant. I have no doubt at all that these are two countable lacks. The grammar of the sentence supports this notion -- she has a lack, one lack that is all her own.
If the sentence read "her total lack of an understanding", then I'd be certain that it was countable. If it read "her total lack of any understanding", then I'd be certain it was not.
I think that this is the uncountable sense. I think that "a total lack of understanding" is closer to "a lack of any understanding" than it is to "a lack of an understanding".
I suspect that we both need to wait for a better answer.
On second thought, I have to assume that this use is uncountable. The countable use would have to include something that implies counting, such as the indefinite article. The counting of "lack" can be implied by the genitive pronoun "her". The counting of this "understanding" isn't implied by anything, and that implies that there is nothing to count.
If this is the countable sense, then we should be able to make a plural form make sense. For instance, "Our understandings are different" seems to be a sensible use of the countable sense. However, if I talk about how both you and I lack understanding, "We both have a lack of understandings" doesn't seem as sensible. It does seem sensible to say "We both have a lack of understanding." In this context, the uncountable sense makes more sense.