Was Milky Way formed after Sagittarius A Explosion?
This is ruled out because that's not what we see when we look back in the past. 

If this was the case at the beginning, we would see these huge stars, with probably different characteristics from the ones formed only recently. But we don't see anything like that. 

From what we see when we look very far away - that's few hundreds of millions of years after the beginning - we see signatures of first generation stars forming, very big stars indeed, but nothing close to what you're talking about.