Hmm... make a plugin that does an UPDATE query on the users table in your database.

Be fairly simple to do. Do you want EVERYONE to be a publisher or only those who selected to be one? That's doable too with a plugin.

The plugin will need to check if they're already a publisher. If they are then ignore the plugin. If they aren't and field XYZ is equal to XYZ then update users table set XYZ equal XYZ where useid equal XYZ.

Something of that sort, but it is doable.

Hmmm.. one probably exists with something similar to what you're wanting, but I do not know of any.

Yes it's possible to set it as a password. Doesn't matter what the input is as it'll do the check based on the databases entry. So it'll read it properly regardless.

If you don't know how then now is a time to learn. Developing a social network takes a lot of time, effort, and work. You can't point + click forever. So grab the documents and read read read. Then play around with making plugins on a local server.