MacOS – How to disable Command + Left keyboard shortcut in Yosemite

Thinking outside the box:

If the only reason you want the keyboard shortcut, is to avoid your mouse, have you considered a trackpoint keyboard?

Because I had RSI in the past, trackpoint keyboards are about the only ones I can use (even at clients, I usually bring my keyboard, or work from my laptop that has such a keyboard).

The big upside of those keyboards is that you don't have to take your hand of the keyboard in order to use the mouse.
For trackpads I still need to (somehow using my thumb is far less accurate on trackpads than using my index finger, o I still need to lift my hand for pointing).

It takes a little while to get used to a trackpoint, but I can't do without them any more.
USB trackpoint keyboards work equally well on Mac and PC.

--jeroen

I've noticed that certain applications seem to trap the gestures, even when they're no longer active.

The only time I've been able to 100% narrow down something trapping various shortcut keys was with either Apple Remote Desktop (the built in screen sharing functions), or Microsoft Remote Desktop.

Even though I clicked outside of the app and started using something else, something caused notably my ctrl+←/→ to stop working.

I unfortunately do not remember the specific steps I took to resolve the issue. Whether it was explicitly sending focus into the remote connection window, and then (with the mouse only) explicitly out of the window, and onto another app that is not just the Desktop, might have fixed the problem. Otherwise, I may have just closed the window in frustration.

Either way, this issue is one that you may have to do some personal diagnosing to figure out. Check for any running apps that are notably not native (Java, AIR-based, etc.), or have some other form of deep integration (trapping keyboard events and sending them to another computer is pretty deep), that may be causing this issue.