They claim that they're legally free and clear, but a lot of the kernel devs are of the view that kernel modules are derived works of the kernel and therefore have to be GPL.
Nope, they just have to be compatible with the GPL. You could, for example, release a kernel module under the BSD or ISC license and be in the clear. The problem is that the CDDL is intentionally incompatible with the GPL.
They may or may not be "derived works" (whatever those are), but they aren't "derivative works", which is what matters as far as copyright law is concerned.
GPL people have a tendency to forget that GPL is a copyright license, and so if someone is doing something that doesn't require copyright permission GPL is irrelevant.
Compliance with a published API doens't mean it's a derived work.
While it obviously can't be distributed with the mainline kernel due to licencing issues, it can be distributed standalone to work with Linux, or any other hypothetical system that provided the same API.
Nope, they just have to be compatible with the GPL. You could, for example, release a kernel module under the BSD or ISC license and be in the clear. The problem is that the CDDL is intentionally incompatible with the GPL.