The correct word to describe two things being combined changes based on context. For example, we would call two metals combined into one an Alloy.
Merger is used most often to describe two companies joining into one. It is jargon from the world of business, and seems odd in any other context.
Fusion is associated with Nuclear Fusion, and is used to make things sound exciting and futuristic. (For instance, there is a popular disposable razor called the "Fusion". Nobody can explain why.) People who want to make a combination of ideas seem powerful and innovative might use fusion to communicate that. When a restaurant serves food that combines two regional cooking styles, it is called "fusion cuisine".
Mixture is the most mundane and common of the words you suggested. When you are following a recipe to make a cake, you will be asked to mix flour, sugar, eggs, and baking powder, then to pour the mixture into the pan. When an artist combines elements of two songs to make a new song, it is called a "re-mix" (but never a mixture).
Interflow is not familiar; It sounds like jargon from a technical profession, perhaps plumbing or metalworking. You should try to avoid jargon unless you are speaking to someone who uses the term.
Alligation is an unusual term which seems to derive from mathematics. A native speaker would assume you meant the legal term Allegation, which means something that is disputed. For example, I might make the Allegation that your thesaurus is producing words a native English speaker would never use.
Embodiment is a common and well-understood word, but it doesn't normally mean a combination of multiple things. An embodiment is something that represents an idea made into physical form. You might say that someone is the embodiment of good taste, or that a building is the embodiment of gothic architecture. An abstract concept that exists in a physical body, is an embodiment of that concept.