There is no sufficient evidence to prove this concept in one direction or another, and there never will be. I will try explain why.
I am not sure where to begin, so I will just do an info dump of points which hopefully will be sufficient, because this is a complicated topic.
Monolatrism is a made up word used to try to discredit Judaism and Christianity. It is not applied to Hindus or the Japanese in which the word might actually make sense. (Everyone has their own ancestors, but I only worship my own)
If I talk about the Greek's acceptance of the Greek pantheon, there is no way to know if I also believe in the pantheon, or if I'm only talking about what I assume other people used to believe. This is because you can never know what my true beliefs are.
Since Abraham and the Jewish people in general (until the spread of Christianity and Islam) where the only Monotheistic religion in the area, there is no way to know if when speaking about the gods of other cultures if they were accepted as real, or used as general concepts regarding the people who believed in them.
Wellhausen and other scholars who demand an evolution of ideas rather than a revelation of ideas can interpret statements to prove their claim, while religious people can also read statements and archaeological evidence to prove their claim. This is because you are talking about the beliefs of the people rather than their actions, and there is no way to know what they thought inside of their heads.
I already said this, but I'll try saying it another way... Let us assume that the Jewish people (since before moses even) thought all other gods were fiction, the same way we believe all TV charachters to be fiction. If someone writes a book, say The Tao of Pooh... is that evidence that the author believes Winnie the pooh to be a real person? The author certainly treats Winnie the Pooh as if he is real, and a super wise monk at that. How are we to know if when the Jews write about foreign powers or other gods that they believe in their existence rather then just using the ideas behind those fictional characters as a jumping point to explain Jewish concepts? In the same way Winnie the Pooh, is used and treated as if he is a real person to help explain concepts of Taoism to Americans.
Lastly, the modern way in which we describe things as truly existing, or being abosultly true, or being completely false are really modern ideas, and this is hard to wrap one's mind around sometimes. The concept of Monolatrism is a new idea (relatively) which tries to pigeon hole ancient minds into modern categories.
The fact that the bible talks about the Jewish people worshiping other deities proves to us that at least some Jews did believe in the other deities existence and even worshiped them... but that makes them Polytheists not Monolatrists.
How does one refute an idea or concept without, at least on the surface, give credence or an appearance of belief about that concept? This is often a criticism of Atheists and the criticism has been turned into a method of argumentation. What I mean by this is that one will argue that atheists do not get emotional about the existence of leprechauns, but they do about the existence of Gd, proving that they do believe in Gd on some level, but don't believe in leprechauns. This has created the bizarre and intellectually indefensible statement that Gd and Leprechauns are the same thing. Wellhausen and his creation of the term Mololatrism has forced the same argument onto ancient Jewish thought.
In my mind, the entire concept is about as useful as the proposition that we are all living in the Matrix.
Monotheism is argued to have started with Abraham, although we know that there was also an individual named Malchei-Tzedek who believed in and worshiped a single deity as well. Jewish tradition teaches that this was Shem, one of Noah's sons.
For those who argue that Monolatrism is real, they argue that Monotheism did not become popular until sometime after Ezra, or during the time of the Talmud. This is part of their general argument that Rabbinic Judaism is a completely different religion than the Biblical religion(s).
