Oh, and I didn't actually answer the questions. So I'll get on that.
1. I think that my main problem with Ebert's definition of art is that he seemed to believe that art was something that should be
passively experienced, for a certain understanding of the word "passive". You look at it, you read it, you watch it, you listen to it, or whatever, but you don't actually
do much of anything. My problem with this is that it doesn't really capture my understanding of how one experiences even the sorts of passive arts that he has in mind. When you read a really good book, you're not passively experiencing anything. The reader visualizes the scene in her head, she might assign voices to speaking characters, and she generally has to do a fair amount of
work in trying to fill in all the gaps of experience a book will necessarily, by virtue of its medium, contain.
Ebert claimed that the key difference was in the notion that games are about choice affecting the person's experience, and art doesn't involve that. But that's not really true, either, is it? For one thing, plenty of games are linear; experiencing the storyline and other content is more or less the same every time a player runs through. How is that different from art appreciation in other contexts, though?
You may see a painting of a bunch of dudes with swords. Okay, that's neat. But you have choices about how to experience that painting. You might try to pick out small details on the canvas. You might acquire contextual information, and find that the name of the painting is
Oath of the Horatii, that it was painted by a man named Jacques-Louis David, and that the painting is classified as part of the neoclassical strain of artistic expression that was most popular in late eighteenth century Europe and associated with certain social views and understandings of history. You could find out more about David's life, speculation from art historians about his choice of subject matter, the specific myth that David based his painting on, and the social context of French life at the time of his painting.
Or you might not do these things. This isn't just fill-in-the-blank stuff, either, because there is no objective way to determine what sorts of things "properly" contextualize any given experience or source. These are choices with no right answer, and there are so many of them that DA:I's reputed "forty endings" pale in comparison.
Experiencing art has always been about choices on the part of the individuals doing the experiencing.
I don't have a particularly good definition of art myself, apart from the good old Potter Stewart standby of "I know it when I see it." (He was talking about pornography, but that's basically the same thing. Isn't it?) So I don't know that I can say that video games are art, because I don't know that I have a good understanding of what art is - not because I don't believe that video games can be art. But I think that Ebert's reasons for his particular definition of what was and was not art were flawed.
2. Again, unfortunately, I don't have an alternative definition of what is and is not art at hand, so I'm logically barred from answering this question.

3. I've never played
Planescape: Torment, and don't have a particularly strong opinion on it or its quality as a game. It might be a slight to bring it up; it might not. I haven't got the knowledge to say.
4. I don't have a definition of art to operate from, but I don't think that genre would have much of anything to do with that definition if I did.