Is it ethical to keep authorized readings after course is over?

Technically, it may not be ethical to keep a case or data set past its license period. Most dataset licenses also specifically state that the holder should not hold material of both physical or digital form (or even preprocessed form) of the licensed matter past its license term. It also stated that the backup of all forms should be deleted.

Having stated above, it is still a widely practised taboo to use cases and datasets past their license periods. You may attain knowledge from it but will only get into trouble if you were to publish any material that relates to the dataset/case that you currently possess beyond its license. This also depends on how strictly the license owner enforces the license.

That is a highly specific restriction, I'm not going to discuss whether or not it is legal and to what extent, but take it as it is. It clearly states the extent of your use of the readings, so after the licence expires you can't use them any longer. That certainly includes presenting them in another course or the same one next year or putting them online etc. But, I don't think there are ethical reasons why you couldn't keep them for yourself, since you already had unrestricted access to that information for some period of time. So in that light, you could've learned them by heart or transcribed them into your notebook by the time the licence expired.

Bottom line, from the ethical point, I don't think the restriction applies for your personal use. From the practical point, you could contact whoever licensed the readings in the first place and inquire the specifics.

The answer is implicit in the condition that you mentioned: the document is authorized for use by you, for a specific period, and only for a particular course. Anything else is prohibited. You may ethically keep the objects (printed pieces of paper) as long as you do not use them in any way whatsoever, which would include reading them. Bottom line, the restriction does not say anything about a personal use exception. As with any contractual matter, you can always ask for a free modification of the conditions, but you must ask.