The elementary OS installation instructions are available here.
I recommend using Balena Etcher to flash the ISO on to the USB.
When you go to boot from the USB, you'll need to press and hold the left Option key after the startup chime.
A menu will appear with attached disks to boot from.
Select the USB drive to boot the elementary OS installer.
This is an issue not directly related to eOS, and so you are probably better off asking at other (more frequented) places, or searching those places for an answer. This post on Ask Ubuntu might be what you are looking for: https://askubuntu.com/questions/217904/unable-to-boot-into-windows-after-installing-ubuntu-how-to-fix
If that doesn't work or would take ...
Yes, you can take a currently-functional Windows installation and convert it to a VirtualBox VM (and probably other VMs) relatively painlessly ... but it'll be done initially via Windows then later on elementary OS (or any other OS, really).
The tool you'll need is disk2vhd. Be sure to get it only from Microsoft as there are bad actors around who have their ...
It's unlikely to happen anytime soon.
Flutter is still a very young framework and elementary focuses on stability.
Since elementary already has a blessed toolchain, Vala, I'm not sure why they would do this.
Changing this would force developers who have already invested in Vala to learn Flutter and Dart and that seems like a lot to ask.
It seems that apps ...