Immaterialism does not open the door for free will though. If an atomic (undividable) event E happens, was E caused by some previous state, or underlying rules? If so, then it's not free will. Was it not caused by any prior events or underlying rules at all (like morality or logic)? Then it's still not free will, it's just randomness.
I think that you are attempting to think about the immaterial in material terms. That's not a knock on you by the way, all of us in the modern world are conditioned by the cultures we are raised in to think along certain lines and it is very difficult to get outside of that and think differently. I struggle with that as well.
If you look at what you say here, you are only allowing two types of causality. Actually really it's only one type of causality, but just varying whether the cause was random or predetermined. Aristotle outlined four different types of causality. In modern philosophy two of those four types are commonly rejected or ignored because they don't belong to the material world. The type of cause that you are envisioning here would probably fall under what Aristotle termed "efficient cause".
You can think of material causality like a chain. A causes B causes C etc. It is possible that some of the events in the chain may be probability based, as you point out with your comment on randomness. True randomness is also not really possible in a purely material system. Randomness is our inability to understand all of the complex and often unknown factors that influence the outcome of events.
So maybe A causes B, but B has a 48% chance of causing C and a 52% chance of causing D. Something like that.
In a purely material system, as you correctly noted, once A has occurred, the subsequent events are inevitable and pre-determined. Even whether C or D eventually occurs is determined from the beginning. But what causes A?
When I talk about Transcendent reality, I mean by that something that exists outside of the material universe. So, if we introduce the idea of a transcendent reality, the things in that reality are not necessarily bound by the same rules of causality that govern the material world. We generally think of those rules as things like the laws of physics, chemistry and so on that govern how things interact. Those material laws are certainly not applicable.
A kind of cause that you don't allow for, because it doesn't exist in the pure-material world is what we might call "purpose". Purpose is like meaning. It is transcendent. It is non-material. This is what Aristotle would term as the "final cause". Purpose is the reason why something is done. The end goal that is envisioned or desired.
If we go back to our chain of causality, A causes B which causes C (or D) that chain has to start somewhere. Namely A. The chain also, cannot cause itself. What I mean by that is that A cannot cause A. So there has to be something which is completely outside of the chain that starts the chain. That outside thing is not bound by the rules within the chain.
Where most people struggle to grasp this is that they think this is just saying, "take the chain A causes B causes C and append another cause at the beginning so it becomes X causes A causes B causes C. The problem with this way of thinking is that it puts X inside the material world as just another cause. In order to properly conceive of it you have to keep X outside of the chain altogether.
Because X is not material it is possible for X to have purpose. X can cause for a reason. X can act with meaning.
Because it is outside, it can insert itself anywhere in the chain even at multiple points in the chain, even after the chain has already started. If there is something outside of the chain of causality, then it is free to interact with the chain outside of the normal rules within the chain. The outside thing can start the chain, it can interrupt the chain, and it can even conceivable change the course of events by interrupting and introducing a different new causal chain.
Another way to think about it.
In your statement you talk about Event E arising from a previous state or underlying rules. Let us assume for the moment that the transcendent reality exists, and let us call it "spirit" because that is traditionally how it has been labeled and conceived.
We can meaningfully talk about states in material systems, but what is the state of a spirit? Does that idea even have meaning? In matter "state" comes down to the position and motion of particles at a given point in time. What does state mean if there are no particles?
There very well may be rules that govern the realm of spirit, but they are nothing like those that govern the material world. The rules of the material world govern how particles move and interact, how forces interact with material etc.
But if there are no particles, no matter, and no forces (as we know them) what would the "rules" be like?
All material objects or material things are made up of parts and systems and subsystems etc. But transcendent things have no parts, no systems, etc.
In short the way that we think about the physical world simply will not work when it comes to thinking about a non-physical transcendent reality. Even analogies are very tenuous at best and usually fall dreadfully short.
Now, leaving all of that aside for a moment, I think ideas should be also subjected to a certain kind of practical test. The specific practical test that I mean is this.. If an idea so radically goes against our experience of life that it literally is impossible for a human being to live out, then it should probably be discarded.
Another way of saying that would be, if applying an idea logically to your daily life would make you either insane or totally ruin your life, then you probably should not accept that idea.
I think determinism (denial of free-will) fails this test. There is almost nothing in our daily experience of life that is more self-evident than free will. It is literally impossible to live without assuming that you do in fact have free will.
There are things in philosophy that run contrary to intuition and sometimes require us to interpret our experience differently that are, in my opinion, true. It is also true, in my opinion, that believing false philosophies (like modernism and post-modernism) can cause us to view the world in ways that are significantly flawed to the point where our view of the world is much like an illusion, or perhaps a dark enchantment.
However, there are ideas that so radically break the world and so radically deny our experience of life and the world that they basically can't be believed by a sane mind. For example, Solipsism, the belief that only you exist, and everything else is illusion, or in some early ancient Greek philosophers who denied the existence of motion or the multiplicity of things.
These ideas may be fun to play around with intellectually but they can't really be believed. If your beliefs about the world end up leading you to one of these ideas, then it is strong evidence, in my opinion, that your belief system or logic or something is significantly flawed.
I would put determinism in that category. The difference between determinism and the ideas I listed above is that those ideas went out of style and the beliefs they were founded on have been rejected. Determinism, on the other hand, is the inevitable logical result of the reigning beliefs of our culture and time. As a result, a lot of people claim to believe it, because to deny it would mean admitting that the entire cognitive framework and worldview of our culture is wrong. But nobody ACTUALLY believes in determinism. What you ACTUALLY believe is determined by how you live your life and no one lives as if they don't exist.
In this regard determinism is much like moral relativism. People are only moral relativists in arguments and when it is convenient to justify something they want to do. No one ACTUALLY fully lives out moral relativism.
So, I actually use the fact that a given worldview leads to determinism as evidence or even proof that said worldview is incorrect. I think that is effective with most people as well.
I love philosophy and I love intellectual life etc. But I will also admit that there are some ideas so dumb that only intellectuals and scholars could possibly believe them.