It's always a good idea to include copyright information when using someone else's work. I'm not a lawyer, but here is my understanding:
Do I have to include the entire license in my GitHub repository?
It depends. If you upload the source or binary of this third party library into your GitHub repository, then the answer is yes. If you are not uploading this third party library to GitHub, then you do not have to include their license in your repository. If you end up distributing the combined overall work (their library and your project), then the 2-clause BSD License says you must list the copyright information and the terms and conditions of the license. This information is usually found in a "COPYING" or "LICENSE" file.
Do I have to credit the developer in my README?
You don't specifically have to credit the developer in a "README" file, but you must include their copyright information somewhere if you decided to send that third party library (in either source or binary form) to somebody else. In that case, I would suggest putting all copyright information in a file named "COPYING" or "LICENSE". Note that providing this copyright information is essentially giving them credit.
If so do I have to credit only the main dev or the whole team?
If you distribute their work, you should credit everyone that has worked on the project, unless they have explicitly forfeited their copyright.
In the case of me distributing the software commercially, can the developer take back my rights to do so?
I don't believe the developer can take back your rights to distribute the software that they licensed under the BSD 2-clause. However, they could choose to change their license (e.g. make it proprietary) on any future modifications/releases of their software. This doesn't mean that you must stop using the software you already have, but it could mean that you might not have the permissions/rights to any future updates.
