Will the current settings cause something to stop working at some point in the future?
Simple answer is I dont know. The official URLs for the mail servers will get updated if equipment changes in the future. Any unofficial ones MAY get updated but its not guaranteed
Would I still need to change the outbound server to relay.force9.net if I didn't renew my broadband contract with Plusnet or would mail.enmail.co continue to work?
Probably , but see above
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.