Depending on how you are using the custom invoice templates, I think I have to winning solutions for you.
If your company just has one custom invoice which is being selected each time that it is being sent out, I would suggest that you change the default invoice in the company file to be the one that you want everyone to use automatically. This would eliminate the need for the unauthorized person to change the invoice format.
If you are creating unique invoices per sales person, that requires more than a single default invoice, I would suggest that you set the default to display the invoice template of the person who does not have access. This way the only person who has to change the invoice template would be someone who already has access to the customization.
If you have multiple unique invoice types across different business lines: you can setup divisions, also called locations, within QuickBooks Online. If the only thing that needs to change is your companies contact information, (phone, address, website, etc) then you could setup divisions within QuickBooks Online Plus. This change will make it possible to select different invoice details based on the division selected.
I hope one of these outside the box solutions helps you solve your challenge! If not, reach out I am always happy to help!