Proof Mind is Fundamental and Matter Doesn't Exist

[Rational AKD]

1. solipsism is possible.
-granted

2. given 1, there is a possible world where mind exists and matter doesn't and therefore there is a property (modal property) of mind that matter doesn't have.
-"if premise 1 were true, then premise 1 would be true"
3. given 2 and according to Leibniz Law, mind and matter are not identical; given 1 and 2, mind is not reducible to matter.
-"if premise 1 were true, then premise 1 would be true"  In a meta solipsist world (possible or actual).....mind would not be reducible to matter - cheifly, because there would be no matter.  Full stop, nothing else need be said...1-3 are -still- contained within premise 1.

you misunderstand premise 3... it is not simply concluding mind and matter aren't the same thing in a possible world that is solipsistic... it is concluding that mind and matter aren't the same thing in actuality. according to Leibniz Law, for 2 things to be identical they can't have properties that distinguish them from one another. if something is possible for A but not B, then they are not the same. and this establishment of non-identical terms would be true in all cases, not just the specific circumstances.

you haven't reworded this, so the problem remains.

I readdressed the problem with substance dualism as per your request. i'm still waiting for your response. EDIT: I missed your explanation in the paragraph so i'll go ahead and address it.

If the world is meta-solipsist, and substance dualism is false - then monistic idealism is true.

your misinterpretation is premise 3 is affecting your summery. let me try: if solipsism is possible, then mind is not reducable to matter. given that and substance dualism is false, then mind is fundamental and everything is derived from mind; thus Monistic Idealism entails.