The answers are hard to find, but in theory, Sgr A* would have formed during the gravitational collapse of the primordial gas cloud that created our galaxy around 13 billion years ago. Regarding the mergers in the Milky Way's history: If galaxies that merged with ours were dwarf galaxies, the formation of supermassive central black holes like Sgr A* would have been unlikely but not impossible.
I think the idea of two large spiral galaxies merging to form an elliptical one comes mainly from theories and simulations of galaxy evolution. But the dwarf galaxies or globular clusters that collided with the Milky Way were a fraction of its size and eventually just merged into the spiral.
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