[This will be one of the many posts that include details about famous effects shots that I have picked up over the years. It is all anecdotal information, believed to be true, but I wasn't there. When this film was shot, I was in a dark room at Robert Abel & Associates writing their raster graphics system.]
4. The exploding head should interact with the set in some suitable way, e.g. brains, skull parts, etc.
I have found a much better trailer, this one for British audiences. Unfortunately, it does not show the head explosion as well, but it is superior in all other ways. On review, I have decided that this is probably a completely different head explosion from that found in the movie. I wonder if it wasn't made especially for this trailer. Some other post will discuss the context of how trailers are made, it is probably different than you expect.