As many as you want, so far it is unlimited per user, but this may change in the future.

Normally it works for everyone, if there is a visual bug just contact support and we will fix it in the updates.

It is a plugin that changes the Theme of the WordPress administrative part.

Yes, you can leave it the way you want by renaming or removing items, the menu will be the same for all types of user (minus the options that are already hidden by the type of user).

Extremely light, all visual edits were made via CSS, only the menu options that contain Javascript for editing, even so users report that the admin has become even faster.