If you're a physicalist, then you do not have free will, but everything is determined already. I suppose your brain just reacts or your choices are determined by preceding events or natural laws. Choice is just an illusion.
That's just wrong - physicalism does not equal determinism.
How does something "physical" (whatever you mean by that) in the brain supposedly create the ability of a person to choose between available options?...What is the something "physical" that supposedly creates the ability to choose between available options?...And given that you presumably cannot answer either of those questions except by "I don't know," why should anyone conclude that the thesis of physicalism is true (or that "the default position has to be physicalism")?
And given that you presumably cannot answer the questions except by "something non-physical that we cannot observe" why should we conclude that the thesis of physicalism is not true?
Define "physical".
It isn't an adjective that any scientific discipline has ever defined or depends upon.
One standard dictionary definition of "physical" is "relating to the natural sciences" - and in terms of "physicalism" that's exactly what it means. "Physical" simply refers to the natural aspects of the physical/material world - essentially matter and energy and their effects. That's what it means.
You defined "physical" as "having material existence," which I noted apparently means "having existence of objects that have mass and volume."
That's an incorrect interpretation of the word "material" and the concept of "materialism" - materialism is essentially synonymous with physicalism but physicalism has the advantage (to the scientifically literate) of being less limited etymologically and therefore less open to misinterpretation (sometimes deliberate) by those who want to support a non-physicalist viewpoint (or agenda).
I stand corrected.
Physical - material and immaterial existance
No you don't - you were already correct.
It's immaterial, but it has a material equivalent in matter.
E=mc^2 Matter and energy can be converted. One equals the other.
That's more like it.
Physical refers to the natural world of matter and energy and the effects thereof. Physicalism is the idea that that is all there is...as I explained in another thread "physicalism is the claim that everything that exists is either physical or an effect arising from the existence, interactions and relationships among physical things".
Now we've got that out of the way - back to the main question of the OP. And I'm going to answer with a question...
If there was a "default position" would there still be a "problem"? Ok we might not still understand the entire process by which mind and body interact and inform each other - but we would know (assuming that we had the right "default position") "where" to look.
But here's the real problem with the options presented so far, as far as I can see...
1. Solipsism is really an extreme "mind-only" idealism - if we can explain the "mind" of our "self" only in terms of "mind" how far does that get us?
2. Idealism suffers the same problem and has no answer to the bigger problem of how "mind" produces such a persistent and compelling illusion of physical reality.
3. Dualism is kind of OK but there is (as far as I can see) no satisfactory explanation as to how "mind" and "body" can interact and influence one another - which is, of course, really just restating the "problem".
4. Physicalism may be the equivalent of the drunk searching for his lost keys under the lamppost not because that's where he lost them but because it is the only place where there is light. We have no idea that's where the answer to the "mind/body" problem will be found - but its the only place we have the tools to look for it.
On balance, I think we have to go with physicalism - and in the process, remain open to the possibility that not all the answers will be found there. On balance, I think the only truthful answer anyone could give at present is "I don't know" - but, on balance, I think the search should continue under the lamppost until we either exhaust all the possible physical explanations or (who knows?) actually find the key.