I may have to go back to the modest days of assembly ... in order to really learn the hardware.
Nah, it's all there and accessible to C; you just have to bypass the ASF functions. And maybe CMSIS as well. Things are to the point where most ARM vendors document their chips with C examples, so you shouldn't really need to go deeper than that.
say I know how to program the ARM to prospective employers.
I guess. When an employer says "I want someone who programs ARM", what they really want is someone who will be able to sit down and start churning out code with a minimal amount of downtime for training/learning. Unfortunately, with ARM, that can range from "can you write a CMSIS library for a bare metal CMx family" to "can you administer an ARM linux distribution" to "can you write apps for Android?" It seems to be pretty common for "can you program in x" to mean "are you familiar with the standard libraries used to build our sort of applications in our sort of environment. If someone asks you whether you know C#; they're not really wanting you to offer a paper on the technical differences between C# and C++; they want to know whether you can write windows apps, including the libraries/etc. (perhaps ESPECIALLY the libraries/etc.)