Hi DogieWogie,
Thank you for reaching Microsoft Community.
I'm sorry to hear about what happened to your Surface Pro 5thgen typecover. Liquid damage is not covered by the Standard Manufacturer's Warranty and the option to fix the issue is for a replacement. Please see this page for details: Get service for damaged Surface. You may also contact support through this page: Contact Us or through our Global Customer Service phone numbers for further assistance.
for the other question though (disable touchpad): Yes, you may disable the touchpad to continue using the device with the external mouse. Go to Windows Settings > Devices > Touchpad. You will have a toggle on top to disable/enable the Touchpad. You may also set it to automatically turn off the touchpad when a mouse is connected by disabling the Leave touchpad on when a mouse is connected option.
Hope this helps.