What evidence could one give of something being possible, without actually demonstrating something to be? If you have already shown something to "be" why would one need the redundant concept of "being possible" when the definition you are presenting for being possible is showing that it IS?
Well, take his example of a pack of cards. Pick a card, any card, from a standard deck. Without looking at it, shuffle it back in. It is demonstrably possible that it was the Jack of Hearts, but it is impossible to show that this is the case.
The trouble is that when we say something is possible we can mean various things, and it's not clear what we're being asked to stipulate in step (1).
For example, it could mean that it's consistent with the way (I think) the world works, but outside my knowledge (as, for example, it is possible in this sense that the card was the Jack of Hearts but not the Hanged Man, since that only occurs in tarot decks).
Or it could mean that it's feasible in principle but known not to be the case (it is possible for me to post on these forums while wearing a cowboy hat, but I do not do so).
Or it could mean that although it is impossible according to the way (I think) the universe works, it is conceivable, like a magician turning me into a frog; that is, it is not logically impossible.
Or it could mean that we're playing along with it for the sake of argument. It might be that if all the terms were defined, and we examined them carefully, we'd find that a "maximally great being" was logically inconsistent, like a four-sided triangle, or inconsistent with some direct observation, and all we mean to say in step (1) if we concede that it's "possible" is: "Yes, OK, let's not be narrow-minded about this ... do go on, Mr Plantinga."
By your terms, NOTHING is possible unless it is shown. How do you know it is possible for you to draw a King of Hearts until you do it? And for that matter you don't know if its possible for you to draw a King of Hearts from a deck of cards ever again. Maybe it is no longer possible. Maybe you can only draw a King of hearts 3 times and then after that it is impossible.
You are basing your definition of possible on having seen it or done it before. That means everything that you have not seen or done before is not possible.
No he isn't, as is clear from his choice of example.