1) You have to make your source available under the GPL if you make a modified version and you choose to distribute that version to the publicNot just "to the public"; it applies if you redistribute it outside your organization at all, even in a private transaction.
You can decline the GPL and still use the program all you want, you just can't legally modify it.You _can_ legally modify it; you just can't redistribute it. A GPL-licensed program gives the user freedom to do _anything_ with it in using it. It only applies restrictions when the software is redistributed.
Just some nits.