Switch to NxtForum

You just open the wallet on your node using a browser and start the forging process from your account. Even if you close the browser the node will keep forging in the background.

If you meant to start automatically each time the node / raspberry starts then take a look to the StartForging add-on.

From the changelog:

The StartForging add-on reads the passphrases stored in a plain text file, as
defined in the property nxt.startForgingFile, one passphrase per line, and
automatically starts forging with each of these accounts when the server is
started. Since passphrases are stored unencrypted, this add-on should only
be used on well secured servers, or with accounts of insignificant value, or
in test environments.

I think you're answering yourself correctly.

Yes, you can run two instances on the same port with different IP addresses using the nxt.apiServerHost configuration property.

But those instances won't be seen as OpenAPI. That's correct. Detecting some edge cases like the one you're mentioning won't be easy. And I don't see much utility about the setup that you describe.

You can use different port numbers and do the trick using the operating system firewall, for example.

Could you please post the full Ardor log?