1. Can I develop RIOT-based C applications using my existing CLion/GCC/CMake/Mac "stack"? Or does RIOT have its own IDE that I should be using on Mac? Is my "stack" a typical dev setup or does it send up any red flags in anyone's mind?
You can use it with you CLion/GCC/CMake stack. I just did exactly that until some weeks ago. Unfortantly my CLion subscription expired and I had to switch to visual studio code ^^
Even though I really like CLion, the RIOT OS extension (https://marketplace.visualstudio.com/items?itemName=attdona.riot) for Visual Studio Code helps in development. It generates the necessary C/C++ Plugin configuration, so that autocomplete works out of the box.
The configuration in CLion is a bit more complext though. You have to do this all by hand. I never did that and just looked into the API doc.
2. My understanding is that my compiler/linker will compile RIOT and my app TOGETHER into the same executable that can be deployed to my MCU -- is this true? What are typical hobbyist deployment pipelines/pathways to get the final executable deployed/installed to the target MCU?
Yep, this is the normal process for firmware development. I mean you maybe could implement dynamic linking on Microcontroller, but the overhead is probably not resonable. It just doesn't make sense, if you work directly on the metal. But just take a look into the bin directory after compiling your source code. You will see that there are a lot of different binaries in there. So, you are also able to ship your application as binary. The LGPLv2.1 license allows you to do that. So, you can ship your app logic as binary, while the RIOT OS core has to be open under LGPLv2.1 license.
My hobbyist development pipeline: I barely think about a pipeline. Even though I write tests. I just have my MCUs here, write some apps, compile them and flash them. There is no big pipeline involved, since I only need one, maybe two devices with the application. For commercial applications this looks probably different and you can make use of the CD/CI pipeline of RIOT OS itself. Just take a look into the Github repos.
The tools are quite advanced. Sometimes not well documented, but an amazing set of tools.
3. How are RIOT apps typically tested? Do I just have to deploy it to the MCU and take the whole system up for a test spin, or are there ways of simulating/virtualizing the test phase?
For the kernel: Write some tests in the test directory. Flash them and check the results. You can, of course, automate this as well. As I mentioned, there are a couple of tools for that. But for the start, I would say just executing them manually is good enough. For your own application: Just take a look into tests and copy the way of doing it to your own application. You are free in what structure you want to use. A bit Makefile magic makes it happen. (https://github.com/RIOT-OS/Tutorials/blob/master/task-01/Makefile)
4. In your introductory examples (wiki) I see vagrant being used. Is this just to simplify the compilation of the source code, or is vagrant/Docker a typical part of the RIOT developer's dev/test flow?
The toolchain for Linux is very simple to install and to use. Windows with WSLv2 also works very well, since it give you a Linux kernel. MacOS on the other hand can make trouble in my experience. Vagrant gives you a simple starting point without worrying too much about the whole toolchain situation. Of course, if you want to, you can use your own toolchain.