(1) Yes, please consider this CC license the current "default" license which governs use of NPL problems which do not have their own license statement.
[Note to existing authors - if you prefer another open-source license, you may of course add a license statement to your problems or to the directory containing the problems.]
(2) That is a good point about attribution.The main point of the attribution clause from the project's point of view is to help manage/measure/mitigate duplication problems in the NPL. As of this summer's PREP course, we have been asking authors who modify NPL existing problems to indicate the origin of the modified problem by adding a comment, e.g., by appending to the header section a comment such as:
## Modified from
Of course, we are only thinking here of modifications of NPL problems for use in WeBWorK courses, and there the "/path/to/original/problem/in/NPL" has enough context to be a sufficient level of attribution. If you have another use in mind, please let us know.
Hope this helps,