What protects issued invoices?
A change applies to new invoices and drafts only. When the owner changes the bank details, Deline first shows how many open invoices keep the old ones: "Issued invoices are never changed."
Can an owner let someone else change settings?
Not the business settings themselves. An owner can give a person Custom access to parts of the books, such as Accounting settings, but business details, billing and team stay with owners.
How do I see who changed something?
In the activity history. See Can I see who changed what in my business?
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.