Once again we are back to "does it matter if it's official doctrine"? The KFD may not be canon, but it is definitely one source.
But those usually relate to the notion of infinity as it relates to Mormon theology rather than how physics relates to Mormon theology. That's not to say there aren't criticisms people make. Given the multiverse, there's no incompatibility between the big bang and Mormon cosmology postulated uncreated spirits. Yet among physicists both the big bang and the multiverse are typically accepted. A big problem with string theory is that it doesn't really make much by way of testable predictions. That's not to say a multiverse is empirically supported yet. String theory and Loop Quantum Gravity are the two main theories attempting to reconcile relativity and quantum mechanics and both include a form of a multiverse. Since the late 1970s physics has been all about inflationary models that postulate a multiverse. In your opinion, is the science of the big bang more or less compatible with creation ex nihilo than so-called "Mormon cosmology" (which I would be delighted to discover is actually not doctrine!)? Why or why not? While a surprising number of things have been contested in Mormon theology over the years, it's interesting that that doctrine never has been. Mormonism has always seen there as being no gap. The theology of creation ex nihilo poses an absolute ontological gap between God and creation. Although the Sermon in the Grove and parts of Abraham 3 also are frequently tied to the doctrine. It primarily comes out of the King Follet Discourse and D&C 93:29. To your specific question, the main argument against creation ex nihilo are that either our fundamental soul or at least the stuff out of which it is made is uncreated and thus co-eternal with God. I see it as a huge strength and makes our religion more like science. I know some see that as horrible and makes religion untrustworthy.
I think one unique aspect of Mormon theology is that given our view of continuing revelation and fallibilism that anything is potentially up for revision.
(2) What is the basis of this "Mormon cosmology," and is it really doctrine?