How else can I prove ownership of the address? I assumed that once I signed and verified the address via the Ledger Live/Electrum method (which was successful) that I needed to send those details (address, message, signing hash) to the person that needs to verify it for proof?
This in theory should be a simple task but I'm on it for two days now.
pair electrum with ledger again in a fresh wallet file and this time choose legacy as the address type when asked. you can begin again via file > new/restore. then you'll be able to generate signatures that can be verified using third party tools. only legacy addresses have a standard signature format.