Asgore never declares war, 6 human kids fall and are taken into the Dreemurr family before Frisk arrives (with Chara within them), and Asriel is revived through a flower but is still a boss monster and everything.
Toriel, Asgore, and the 6 kids get shattered across time and space, W. D. Gaster seizes the throne in their absense and becomes the new ruler. Sans and Papyrus aren't around at all at this point in time.
How are Undyne and Alphys' lives changed by this?
Alexander Barnes
Alphys becomes the viceroy of science and takes a Gaster dicking. Undyne wouldn't have a reason to kill humans really unless Gaster said the same shit Asgore did, I'm not sure what would happen.
Ryder Flores
i wanna smooch him all over i wanna get covered in his pollen
Jace Ross
Alphys discovers that the multiverse is no longer stable and that each child came from a separate universe. Each much be returned to their own respectful timelines so she teams up with Undyne and produces a high-tech armor that can jump between realities however is malfunctions and ends up in Frisk's timeline where Undyne seeks their help.