"Why do you need an AR15?"

[stanc]

If the 2nd Amendment was meant to be an individual right, why did it not follow the example of the Pennsylvania and Vermont constitutions, and explicitly cite self defense as a reason, instead of only defense of the state?

Considering what the 2nd Amendment actually says, and taking it in context with the Militia Act of 1792, I have to agree with the following comment that I read elsewhere:
“It is therefore entirely sensible that the Second Amendment’s prefatory clause announces the purpose for which the right was codified: to prevent elimination of the militia.”
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It is called the Bill of rights, not The Bill of Powers. Are you saying the Bill of Rights are not individual rights?

I'd say it is partly a Bill of Restrictions on the Federal Government, and partly a Bill of Individual Rights. But, I was discussing only the discrepancy in the 2nd Amendment noted above.

Individuals have rights, government has powers.

States apparently also have rights. https://constitution.laws.com/states-rights

And according to the 10th Amendment, the people have powers, too.

Your argument falls apart if the militia is other than individual citizens who make up the whole of the militia.

That makes no sense. Individual citizens make up the whole of any organization. Did you mean to say that the militia was (as per the Mason quote) the whole people? If so, there are some flaws with that idea.

The "whole people" make up the unorganized militia, which in reality is not actually a militia per se, but a reserve manpower pool. Since it is not required to conduct military training, and is not organized into operational units, it cannot be the "well regulated" (i.e., trained, disciplined, and organized) militia addressed in the 2nd Amendment.