Illegal property values + sorting
- Indeed, currently only things shown on the help page for datatype date are possible so this would be a feature request. Probably you could recreate an example setup at sandbox.semantic-mediawiki.org and link to this when creating and elaborating the feature request at github.com. Personally I cannot assess if it will be possible to support partial dates such as year-month or month-day.
- Perhaps you could additionally set some kind of "helper" property which is automatically set in a way that it allows sorting, e.g. a property of type text: So a date like January 1, 2016 could be 20160101 or January 1990 would be 19900100 etc. and sorted accordingly afterwards.