Thanks for the write up and I think I learned a lot of stuff. But at the same time I think you’re getting a bit hung up on the particular word “diamondoid”. It’s normal in language for words to have different meanings in a base-level language versus in particular specialized jargons. For instance “speed” and “velocity” are synonyms in normal English but in physics jargon they’re distinct. As Kuhn famously noted whether a lone helium atom counts as a “molecule” or not depends on whether one is speaking chemistry jargon or physics jargon (with general English not being precise enough to give an answer). And very few people would consider carbon a “metal” but that’s just what it, along with every element heavier than helium, is called in astronomy jargon.
In general, in English, if you have a base word you can append the suffix “-oid” to create a new word that means “something similar to the base word without being it”. So you might properly call a bean bag a “chairoid”, something with many important chair properties without being a chair. And often these constructions get taken up and given precise meanings in different formal jargons for words like “sphereoid”, “planetoid”, or apparently “diamondoid”.
As general practice I think it’s good to avoid stepping on precise meanings in jargon when you’re aware of them and there’s another word that works as well. Use “speed” rather than “velocity” and call a rhombohedron “cube-like” rather than “cuboid”. But at the same time I don’t think it’s a very bad mistake to use a word that makes sense in non-jargon English in a way that conflicts with established jargon. And if someone uses a word in a way that doesn’t make sense in jargon terms but does in ordinary English usually its best to go for the non-jargon interpretation.
I think it’s wrong to think of using a construction obeying the normal English rules of word construction as creating a new term. If I said that a person was “unwelcomable” that wouldn’t really be inventing a new term despite the fact that it doesn’t appear in a dictionary. It’s still a normal English word because it’s a normal English construction.
Yes, diamondoid as referring to the adamantane family might go back to the 1960s but in practice how many people understand it that way, 100,000? In theory everyone who has taken high school physics should understand the difference between speed and velocity but as people use it velocity is still most most commonly used as a synonym for speed and I think it’s useless to try to police that. And likewise “-oid” constructions that happen to collide with some field’s technical usage if the construction isn’t used in the formal context of that field.