"z" Key not working, Command key also

See if an external USB or Bluetooth keyboard works. If it does go into the Keyboard control panel and click on the 'Input Sources' section. See what your internal keyboard is on the left is it U.S.? If not correct it.

You can also add in a second keyboard and delete the original one then add it back in to refresh it.

You may also want to use the iconic view of the keyboard to double check if the key is working correctly. Under the 'Keyboard' section check off 'Show Keyboard, Emoji & Symbol Viewers in menu bar'. Once you've enabled it go to the upper right of your menu you should see a icon with the command key. Opening the keyboard viewer will allow you to see the pressing of the keys as you type on the keyboard see if the key has failed.

Let us know what you find.

Sadly this is a big job ;-{

You have two ways to do this, either replace the complete upper case while expensive it is by far the least amount of work. Or, take apart the upper case so you can replace the keyboard subassembly. While more complex if you're not careful you can damage the battery in the process.

Here's the IFIXIT guide you'll need to follow for either: MacBook Pro 15" Retina Display Mid 2015 Upper Case Assembly Replacement.

Here is the upper case part: Uppercase w/Battery for Mid '15 MacBook Pro Retina - Apple P/N 661-02536

Here is the keyboard part: Macbook Pro 15" Retina A1398 Keyboard backlight 2012 2013 2014 2015 You do want to get the one with the replacement rivets or screws.

And lastly, if you need a fresh battery: MacBook Pro 15” Retina A1398 Battery.

Here's a vid showing the process of replacing the keyboard: Macbook Pro 15" Retina A1398 Change Keyboard

You might want to see what it will cost you for the Apple Store to repair your system. Sometimes they can do things cheaper and offer a warranty to boot!

Good Luck!