Meanwhile, back at the ranch:

Was singing as mandatory in the merchant fleet as it supposedly was banned in the Royal Navy?

What was an instrumental fife or fiddle capstan cadence called in the merchant fleet? Was it also considered a shanty? I've never read anything to suggest otherwise.

If the merchant's instrumental is a shanty why then does it cease to be a shanty when done just like that in the Royal Navy?

Most of the planet doesn't do English as alchemy. It's just another language to sing in, or not.