Does G-d decide who dies? I am having difficulty resolving this. For example, in a multiple tragedy where five innocent teenagers die in a car accident or any other such accident, if G-d decided all should die, shouldn’t He be considered responsible for all the anguish related to this?

Hashem is responsible for everything happening in this world and outside it. That does not mean that we can "blame" him for bad things we humans do. If one drives carelessly and is killed in an accident – he can only blame himself. If a careless driver kills someone else by accident – it's the drivers fault not G-d's. Just like one can't blame anyone but himself for lung cancer after smoking a packet a day. The master plan is all taken care by G-d but the players in that game, i.e. us – we have the free choice of doing what we want and we should take full responsibility for our actions. (see more in the writings of the Ramchal "Derech Hashem" and "Da'at Tvunot") The Rambam in his book "Guide to the Perplexed" (chapter 12) explains that most of the suffering of the world is man made. Sometimes Hashem intervenes in the world with a miracle or divine intervention to save the righteous innocent, but only sometimes. The ultimate reward for our good deeds and punishment for our bad ones is in the spiritual world we enter as we pass away from this one.