I'm pretty sure it works this way.

Valve pays the US some taxes out of our profits, using our EIN. Then the IRS tell the Australian ATO how much money they have received and it is deducted from our taxes in AUS. The Australian tax treaty means that Valve only have to pay 5% to the US instead of 30%, then we pay the rest here.

The engine has had a few different codenames, but the closest to an official one is just ManicEngine. Some other things it's been called over the various iterations have been TeaRacer, Gumball, CubeWars and Moodcubes, usually to do with whatever prototype forced an engine rewrite.