Why did George Kirk's death and the destruction of the USS Kelvin cause so many differences in Abram's Star Trek Universe?

It is reasonable for the death of George Kirk in Star Trek (2009) to affect James Kirk's childhood, leading to James being a troubled teen and not joining Starfleet until he was a 20-something.

But what is the reasoning behind all of the starships looking drastically different in this divergent timeline?

Whose death on the Kelvin lead to Spock starting a romantic relationship with Uhura?

What event lead to Admiral Marcus finding Kahn and his crewmates instead of Kirk and the Enterprise?