I'm trying to parse out your meaning. If I split it into two sentences, I get:No, it falls to Starfleet to defend the Federation, not Earth.
It falls to Starfleet to defend the Federation. It does not fall on Starfleet to defend Earth.
-or-
It falls to Starfleet to defend the Federation. It does not fall on Earth to defend the Federation.
Not sure which way you meant that. Either way, I would argue that it is up to Starfleet to defend Earth, and up to Earth to defend the Federation. Humans built the Federation. It was their idea. They brought all the other planets into the fold. Earth is the glue that holds everything together.
What's the saying? As Rome goes, so goes the Empire. So true is it for Earth and the Federation.
Can the Federation survive without Earth? Perhaps. But I believe it would be a hollow shell of what it is now.
In my view, it would depend on the era. During the TOS-era, I'd say that Starfleet was 99.99% human. It was mentioned that Spock was the only Vulcan to attend Starfleet Academy. It appeared that everyone on the all-Vulcan ship went to the Vulcan Science Academy. Even in the TAS series, there were few non-human crew members. Ditto in TMP. Only from the second movie on did we start to see more and more non-humans in Starfleet, but it was still a majority of humans.And Starfleet, like the Federation, is made up of more than one species than just Homo sapiens.