the error you are getting is occurring because you have bound the DatePickerID column in the database to the WADAInsertRecordID form element, which does not have a value.
the DatePickerID column in the database should be set to auto increment, you should not be binding that column in the insert behavior, you should let the database give it a value.
"On a tangent: On the WA single record insert form, why does the form not have a way to browse for a session variable? "
it does, if you click the lightning bolt icon, you can expand the session collection of the bindings panel and select any session bindings that have been defined.