Lost bluetooth connection with mouse
This shouldn't make a difference since you're talking about an Apple Magic Mouse 2 and not a 3rd party Bluetooth mouse, but one thing I have seen that does work in some cases is to make sure the mouse is the first Bluetooth device paired to your computer. 

To do this you will need to reset both the macOS Bluetooth Device List and Bluetooth Controller. However, this will reset all connected Bluetooth devices, including your external keyboard and headphones.

In other words, this will remove all existing pairing information for Bluetooth devices, so each will need to be reconnected.
I have some steps you can follow, but I strongly suggest you read all of the steps first before commencing (or perhaps print out the instructions / view them on another device).

Before starting, ensure you have the Bluetooth icon showing in the menu bar. If it is not there, open System Preferences > Bluetooth and enable the option Show Bluetooth in Menu Bar.

Also, since you're trying to resolve an issue with a Bluetooth mouse,  you will need to use your built-in trackpad to follow these steps:


Hold down both the Shift and Option keys and at the same time click on the Bluetooth icon in the menu bar. Once the menu is showing, release the keys.    
From the Bluetooth menu, choose Debug > Remove All Devices
Hold down both the Shift and Option keys again and at the same time click on the Bluetooth icon in the menu bar. 
From the Bluetooth menu,  choose Debug > Reset the Bluetooth Module
Shutdown your MacBook Pro
Ensure your Magic Mouse is fully powered off
Restart your MacBook Pro
Connect the Magic Mouse to your MBP via the USB cable
Now switch on your Magic Mouse
Now click on the Bluetooth icon in the menu bar and choose Open Bluetooth preferences...
When the Magic Mouse pairs with your Mac, it appears in the list of devices
Exit System Preferences


Now test the Magic Mouse long enough to see if it works okay. Once you're satisfied it is, you can go ahead and pair your other devices.

Let me know how you go.