How can I add keyboard shortcuts and mappings to OS X (for example: I'd like Ctrl-u and Ctrl-i to map to [ and ], respectively)
Solution 1:
For this task (and other remapping even with makro support) I recommend using Ukelele which is a Unicode Keyboard Layout Editor for the macOS.
Yo define your own keyboard layout and just use it via the system -> keyboard layout chooser.
Ukelele is freeware and more details can be found on the download page