Don't worry, you are making progress. These problems don't go away overnight, horses are not like a video game where you insert the right code and your problem is gone. Instead, horses go through a series of defensive patterns each time a little less extreme, until they run out of defenses and reasons to use them.
Remember, the are fight or flight animals, but all have a full deck of cards. The defensive pattern that you were dealing with was aggression. As long as the aggression was working for him, he had no reason to change it. When you started working to break the defensive pattern, he jumped to the other extreme, flight. Naturally, flight inside the stall is a little difficult, so he "flew" in a different way through introversion. He basically retreated inside of his body, avoiding the situation (like he would if running from it) unlike directly confronting it like he did with his fight patterns. This is why you get the "time bomb" feeling. If you close in to quickly on a horse in this state, an explosion is exactly what you get.
What you do is exactly as kv said. I personally have sat in horses stalls for over an hour with a book or magazine until they relax. Once they do relax, sometimes I won't even approach them, I just leave. He will lick, yawn, and chew, you may just have to wait a little bit. I was working with a paso with extreme introversion issues in the stall that took over an hour to relax the first time I worked with him. Within a week he was meeting me and every other person at the stall door with direct eye contact and ears forward. Without that attention, it is very difficult to get work done.
Once Ice realizes that "flight" won't work either, don't be surprised if he resorts back to aggression again, its normal, but shouldn't be nearly as intense as it was the last time. You will gradually get through these until he runs out of defenses and runs out of reasons to use them. Your attention to the situation can make the progress go very quickly, as the more attentive and consistent you are, the stronger leadership skills you will be showing which is all the more reason for him to not be defensive.