The custom CSS styles you've created are not applied on admin side because some CSS properties could mess up the whole Gutenberg admin layout.
That's why we've added a color to identify the style
Yes and that's really easy!
JU Translation is a translation tool included in all our WordPress plugins: get, share and manage your extensions translations from a visual and easy to use interface! More information here:
The impact is minimal as we using to the maximum the tools provided by the native Gutenberg editor. Note that Gutenberg will be loaded with WordPress whatever you do so it will have less impact than any other page builder.