Upon review of the APIs, I think ideal to the user is to remove these current "sorter/comparators" so that when you want to use sorting mergepolicy, you just pass it a normal org.apache.lucene.search.Sort.
I know it seems a little crazy, but IMO the logic is duplicated. So someone should just be doing:
Sort sort = new Sort(new SortField("field1", SortField.Type.DOUBLE), new SortField(....));
iwc.setMergePolicy(mp, new SortingMergePolicy(sort));
This would let people be able to sort in reverse, by doubles/floats, by a combination of fields, expressions, whatever. And would deconfuse the API.