Hello Wanling,
1. There are currently no plans to support this, the only way for collaboration currently in Web IDE is using Git.
2. Also there are no plans for it currently, the functionality is similar to the UI5_REPOSITORY_LOAD provided in ABAP.
3. Using team provider has many benefits in it, and it's true there are features missing when comparing it to the deployment in Web IDE.
When using Web IDE there are several files meant for design-time, like the project.json for example that are added to the project upon creation or import. These files are critical for Web IDE as we use them for other features and more. Deploying them will not have any affect.
Regards,
Michal.