Both RevitPythonShell scripts and Revit Python Macros are relying on Iron Python. In both cases, at least in Revit 15, neither require the installation of IronPython. I believe RevitPythonShell installs with the capacity to process the ironPython script (rpsruntime.dll). Revit must install with the same capacity.
Therefore, I assume the two programming environments are being executed from separate sources. Is this accurate?
How would I be able to install an external module as I would typically use pip for? Another words, if I wanted to use the docx, pypdf2, tika, or other library - is this possible?
I agree with your assumption that these two environments are probably completely separate and independent of each other.
Since the Python code that you write within them is pure .NET, it is easy to reference other .NET modules, assemblies, libraries, whatever you want to call them. They would reside in .NET assembly DLLs.
Using pip to add external Python libraries is a completely different matter, however, and rather removed from the end user environment offered by these two.
To research that, you will have to dive into the IronPython documentation and see how that can be extended 'from within' so to speak.