No, it's based on a trust system so larger teams can support my work further.

The features and enhancements are only available in the Armature Builder and do not affect vanilla Rigify. This way you're able to separate workflows depending on your current needs.

No, as it's not a proper rigging tool. Rather, it can be considered an "armature translator", meaning that you need an already skinned character to utilize Retargify. However, this method allows for skinning to be easier to work with and more transparent as you're always able to tell which bone controls which area, without having to go through hidden bone collections, or having to redo the skinning process if your character already has skinning data. 

You can still benefit from it as it shortens the time it takes to build an armature as much as possible, and allow for extensive experimentation without risk of losing animation data. This method, while different from the standard process, gives you more room for expansion should you ever need it, without losing any features.

Yes, they can use it with all Rigify features available with the addition of the baking tools. However, they will not be able to modify the armature without having the plugin installed.

It does not modify the armature structure whatsoever since it works with constraints. This keeps compatibility with the software they may belong to, allowing for new animations to be added in without breaking previous existing ones.