Is buying a game from a company that produced a game offensive to Islam considered disbelief (kufr), or is helping someone in a disbelief-related game considered aiding in disbelief or sin?

You do not become an infidel merely by purchasing a game from a company that produces games offensive to Islam. However, it is preferable to refrain from buying from them. Helping a person play a blasphemous game is unlawful (haram), and playing it or assisting in playing it may reach the level of disbelief (kufr) if the heart assents to disbelief.