The entry for Melta Bombs (p144) specifically says they cannot be thrown at all, however on the Grenades and Explosives table (p143) the melta bomb not only has a throwing range, but also the scatter quality. So can it be hastily and awkwardly thrown, or not thrown at all?

My personal feeling is that it shouldn't be thrown at all. It's a shape charge, you don't throw those. However, if the decision is made to make it a thrown weapon, I would make it scatter even more than other grenades since it's heavy and awkward to aim precisely (and as a disincentive for chucking melta bombs all over the place).