MacBook – When mirroring the Macbook Pro with an ultra-wide external display, can the resolution be set to the external’s native resolution

You can try Display Maestro :

Unlock all display resolutions Display Maestro gives you full control over attached displays, allowing the usage of all available resolutions and bit depths. This is done by ignoring the operating system setting of hiding potentially unsafe resolutions.

Restore legacy depth modes It is ideal for running legacy games which require 256-color mode and do not automatically switch the screen depth for you. Mac OS X 10.5 Leopard removed the ability to set 256 color mode manually using the built-in preferences pane, but the feature is now available here in Display Maestro!

Light-weight and background-friendly Display Maestro is a small application which can be configured to launch during login, and sits quietly in the menu bar. Upon exit, it will also automatically revert your display back to the resolution chosen before it was launched.

You ABSOLUTELY can do this.

The statement on the box is wrong. You can use either two mini-display port to displayport cables, or two Thunderbolt cables.

My statement was based on the fact that you can (according to Apple's documentation) run two Thunderbolt monitors on that laptop, and the available resolution on the Thunderbolt cable is clearly capable of 3440x1440.