I'm not in the mood for some serious reading so I only went through the first paragraph, but just reading that far gave me reason to believe that the writer had misunderstood a lot points with the arguments involved, including Plantiga's use (or misuse) of modal logic.

The famous rule of modal logic is:
If something is possibly necessary then it is necessary.
What this rule states is that something is either necessary or it isn't, and if it is necessary then it makes no sense to talk about possibility.

There are two ways to criticise Plantiga's argument.
1) He is begging the question.
As 'possibly necessary' is basically the same as 'necessary', to use 'God is possibly necessary' as premise is to use the conclusion 'God is necessary' as a premise.

2) He is equivocating.
Mathematical truths are necessary truths.
"2 x 2 = 4" is necessarily true - there is no possibility about it.
If "2 x 2 = 4" is possibly true then it is necessarily true.
This is the same for simple equations as it is for more complex ones.
However, for more complex ones, although the truth of the statement is 'necessary' from a logical point of view, if the problem hasn't had a full logical analysis then we don't know what result the logic will give us. In this context we can use the word 'possibility' as we have not yet determined what results our logical analysis will give us.
This is the 'possibility' that Plantiga uses for his 'God is possibly necessary' premise because it is possible that a logical analysis of God would still find him to be a 'necessary being' as the question is open. (I don't consider this question to be open btw, but I think that many do.)
As you can see, this is a different use of the word 'possibility' compared to the 'possibility' as used in modal logic. So to say that 'God is possibly necessary' in this sense to try and say that 'God is possibly necessary' in the modal logic sense is to equivocate on two different uses of the word 'possibility'.

This second criticism of Plantiga's argument is what is important here as it shows an understanding of modal logic and why the premise "God is possibly necessary" cannot be justified in any way other than the definition of 'God' itself. It certainly can't be justified by experience as if something is logically necessary then it's truth is determined prior to any experiential evidence. That is the meaning of the word 'necessary' in the context of modal logic.

Maybe I've been unfair to the guy and judged him on the opening paragraph rather than read his entire essay, but given my understanding of modal logic, the only possibility I can see is that he's failed to understand some vital part of it.
If what I've written here about modal logic is correct then the argument he has sketched in that opening paragraph is doomed to fail. If you disagree with my comments on modal logic then argue back, and if I find that my reasons for dismissing his essay are flawed then I will give it a more thorough read.
Surely that's a fair deal?