This started as a reply to The Ancestry of Fantasy but it only answers the first line of that posting, not the main question, so I moved it here.
This is more a comment than an answer, but I need more characters and the ability to do some formatting. I believe you're using the label alternative world in the wrong way, which makes it difficult for me to answer some of your questions, like this one, this one, or this one. You might note that it takes five votes to close, so I'm not the only one who has problems with the questions.
Speculative fiction can be divided into subgenres. The borders are fuzzy, but alternate history has a generally accepted meaning. Alternate world is less clear. All are appropriate topics for Worldbuilding, BTW.
A story involving magic and the classic fantasy races. Tolkien, swords and sorcery, fairy tales, and all that.
- Urban Fantasy takes the magic into the modern world. Still fantasy.
- Horror is often, but not always, using fantasy tropes.
A story that explores the impact of scientific discoveries on the protagonists. Often with a focus on engineering, and bordering on the non-scientific where it comes to FTL drives and the like.
- Space Opera. Starships and heroes. Star Trek and Star Wars, even if the latter borders on Fantasy.
- Technothriller. A science fiction story set in the near future, often involving espionage or weaponry. Tom Clancy, Dale Brown, much by Michael Crichton.
- Time travel. A science fiction (or fantasy) story where characters travel into the past. There they may or may not change history. Poul Anderson, SM Stirling.
- A story which explores the outcome when a historical event turned out differently. It should be an outcome that could have happened, but didn't. That means no time travelers. No aliens. No elves.
Summary: The label "alternate Earth" generally refers to alternate history. The questions you're asking seem to go in a different way. The definitions are not set in stone, but the links show that my usage isn't completely without precedent.