My Macbook Pro is acting as if the SHIFT key were pressed when it absolutely isn't This started happening suddenly yesterday, and it's annoying/making my life impossible, to say the least.

I've tried searching for a solution but every solution given points toward a hardware issue. However, I'm sure I don't have any. I've checked in pages such as http://keyboardchecker.com and http://whatkeyamipressing.com and the SHIFT key doesn't appear as pressed—unless I actually press it, and when I release it, it shows as no longer pressed. However, my MBP still acts as if I were holding on to that key for dear life!

I use a USB keyboard, and have done so for a long time; nothing has changed in that respect, so I don't know what could be going on. I will say it's a Logitech MX Keys and, if you know that product, you'll now it's the buggiest thing in the world! So I disconnected it to see if perhaps a happy firmware update was culprit, but the issue continues with the USB keyboard off and/or disconnected.

Because this only leaves the internal keyboard as the possible culprit, I've tried disabling it to see what happens but apparently in MacOS Sierra 10.12.6 you can't do this. I've tried disabling it following instructions given in this forum. I've entered the following into Terminal: 

sudo kextunload /System/Library/Extensions/AppleUSBTopCase.kext/Contents/PlugIns/AppleUSBTCKeyboard.kext

All I get is: 


  (kernel) Kext com.apple.driver.AppleUSBTCKeyboard not found for unload request. Failed to unload com.apple.driver.AppleUSBTCKeyboard - (libkern/kext) not found.


And although it says you probably get an error but the keyboard is disabled anyway, I'm not seeing that: my internal keyboard still works fine and the issue persists.

Elsewhere I also read the workaround/solution for when this happens was to press SHIFT and, simultaneously, press CAPS LOCK several times (5-10 times). But this doesn't work either.

I've also checked, and Sticky Keys isn't enable. I've enabled it, pressed SHIFT 5 times, pressed SHIFT 5 times again, disable it again, etc., and the issue persists.

The only thing that's changed in my setup is that a week ago I added an external monitor connected to my MBP through the HDMI port. However, this was a week ago and the issue I speak of started yesterday.

What solves it? Well, clearing my Mac's cache folder and rebooting it seems to be the only way to fix it. That is, until it comes back late run the day.

Has anyone run into something similar? ANY input will be highly appreciated!

P.S. As you can see, this text has been written in normal sentence form, i.e. not entirely in Caps, which is weird, right? Because I'm having the issue right now, as I'm writing this message. So how do I know that my MBP thinks the SHIFT key is pressed? Well, when I click on a link it sends it into my reading List instead of opening it (to send stuff to your RL you click holding down SHIFT, of course); when I click somewhere on this text, for instance, it paints the text from the last place the cursor was to the place I've clicked, as if SHIFT were pressed; when I click on 2 or more files on my desktop it selects them, as if I were group-selecting them pressing SHIFT; etc. 

P.S.S. Nor does my computer boot in Safe Mode when restarting. When I restart it starts out fine; it's later on it become wacky.
In your case, when you reboot and if it has a shift button press then it should boot in SAFE mode. As you are not facing this, I am sure this is not a full hardware issue. So NVRAM reset should help because temporarily you are able to fix by a quick reboot.

Reset NVRAM or PRAM on your Mac

NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. PRAM (Parameter RAM) stores similar information, and the steps for resetting NVRAM and PRAM are the same.

Settings that can be stored in NVRAM include sound volume, display resolution, startup disk selection, time zone, and recent kernel panic information. The settings stored in NVRAM depend on your Mac and the devices that you're using with your Mac.

If you experience issues related to these settings or others, resetting NVRAM might help.

How to reset NVRAM:

Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command,P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.

More detail from the Offical Apple Support page