I am not sure about the part 2. I could be wrong, but I believe V-Singular announced that TWC was the last DLC mission pack. If I remember right, he also said there would be more DLC packs, though, which I am going to guess that he means clothes items, vehicles, weapons, and that sort of thing.
Oh and floatmill, I too thought that the shooting part of the first mission was absurdly difficult. I was literally shooting everything in sight and still my car kept blowing up over and over again right near the end. I have the feeling that either that mission was not properly beta tested or perhaps Volition devs were a little self-conscious about the shortness of the mission packs and artificially over-inflated the difficulty of that section in an attempt to make the pack seem longer. That is, of course, pure speculation on my part, though.