It was my understanding that the only reason all these terms exist is because Hohner still have a trademark over ‘melodica’. Much like in the world of those UFO-like, hand-played metal percussion instruments that were called ‘hang’ by the people who invented them, who were fiercely proprietorial about the term when other people developed similar instruments (and yet, from what I’ve read, also resentful when the community of people who play those things coalesced on ‘handpan’ as the generic term for the hang and its imitators – it strikes me that you can’t reasonably have it both ways).
So I will keep on using ‘melodica’ as the generic term, unless and until something better comes along. Though I’m not quite sure where the borders lie. Does the 3-row chromatic Accordina count? I’d say probably. The Claviola? Maybe. The Harmonetta? Probably not. It’s a slippery slope 🙂