I'd asay you're right with the spells. But about the frozen Mallet look at other guids they all takes it too.

Frozen hammer is a bad item on kogmaw. Kogmaw has insane range if you couple that with ghost and flash (or any GOOD summoner spells) you are very hard to kill. Additionally kogmaw is a carry he needs to focus on damage items not tanking items. Increased attack speed is greatly superior to extra health on a champ that wishes to deal the majority of a teams damage.
Additionally the way you level your skills is seemingly illogical. Kogmaw is a dps champion and would benefit much more from maxing caustic spittle sooner. I would recommend one rank in void ooze for its slowing ability and then maxing caustic spittle. The additional armor pen and attack speed is a better than a hundred AOE damage.