Is there any library, preferably in python but at least open source, that can summarize and or simplify natural-language text?
Maybe you can try sumy. It's a quite small library that I wrote in Python. There are implemented Luhn's and Edmundson's approaches, LSA method, SumBasic, KL-Sum, LexRank and TextRank algorithms. It's Apache2 licensed and supports Czech, Slovak, English, French, Japanese, Chinese, Portuguese, Spanish and German languages.
Feel free to open an issue or send a pull request if there is something you are missing.