And yes, it's worth pointing out that in many first world countries (esp. my own), many higher density neighbourhoods actually have relatively high environmental footprints - usually because they're quite affluent ones. But also because they're often occupied by single people who don't have as many opportunities to share/pool resources with others as those in suburbia. So we need to realistic about what could be achieved simply by changes in government policy around subsidisation of various modes of urban development.
Yes An no.
Yes it is already built.
No because there is a lot that can be done to improve existing infrastructure.
Public transport can be retro fitted rail corridors, dedicated bus lanes, bike only paths, walking infrastructure
Zoning can achieve a lot too. Allowing more commercial pepperpotted amongst residential
You do have to defeat many vested interests, but unless we defeat them they will burn the world for profit, so we must defeat them
A real test for democracy