(1) If you accept their contributions and distribute your project under the MIT license, your project is distributed under the MIT license.

(2) There is no implicit copyright assignment. The author of the change retains copyright in that code, and you retain copyright in your code. You may want to look in to copyright license agreements in which you get your contributors to agree to allow you to use their code in your project and maybe agree not to revoke that permission.

Boilerplate headers have no real legal standing: copyright applies the minute you type the text of the source code and is only worth what you can prove in a court of law. Should you wish to change the distribution licensing at some point in the future, you will need to secure the permission of all copyright holders. In the absence of copyright assignment or copyright license agreements that means everyone who has ever contributed a line of code.