There are cases where a suitable ancestor in the tree cannot be found, or a few docs are missing due to race conditions. To mop these up, we make a penultimate pass that considers every unique workflowGuid, preferring the locale with the most docs as a basis but falling back if necessary to any in which it exists. And in the final pass, we use an aggregation query to find absolutely every last workflowGuid that does not have every locale.
function fallback(callback) {

      return async.series([ viaBasis, noBasis ], callback);

      function viaBasis(callback) {
        return self.apos.migrations.eachDoc({ workflowLocale: basis }, 5, function(doc, callback) {
          if (!self.includeType(doc.type)) {
            return setImmediate(callback);
          }
          doc.workflowResolveDeferred = true;
          var afterSaveOptions = _.assign({}, options, {
            permissions: false,
            workflowMissingLocalesLive: self.apos.argv.live ? true : 'liveOnly'
          });
          return self.docAfterSave(req, doc, afterSaveOptions, callback);
        }, callback);
      }

      function noBasis(callback) {
        var localeNames = Object.keys(self.locales);
        var orphans;
        return async.series([ find, fix ], callback);
        function find(callback) {
          const query = [
            {
              $match: {
                workflowLocale: { $in: localeNames }
              }
            },
            {
              $group: {
                _id: "$workflowGuid",
                count: { $sum: 1 }
              }
            },
            {
              $match: {
                count: { $lt: localeNames.length }
              }
            }
          ];

          return self.apos.docs.db.aggregate(query).toArray(function(err, _orphans) {
            if (err) {
              return callback(err);
            }
            orphans = _orphans;
            return callback(null);
          });
        }

        function fix(callback) {
          var seen = {};
          if (!orphans.length) {
            return callback(null);
          }
          // The aggregation query returns the workflowGuids but I haven't been able to
          // convince it to give me representative _ids to go with them, so we will just
          // ignore the additional locales after we fix each one once. -Tom
          return self.apos.migrations.eachDoc({ workflowGuid: { $in: _.pluck(orphans, '_id') } }, 5, function(doc, callback) {
            if (seen[doc.workflowGuid]) {
              return setImmediate(callback);
            }
            seen[doc.workflowGuid] = true;
            if (!self.includeType(doc.type)) {
              return setImmediate(callback);
            }
            doc.workflowResolveDeferred = true;
            var afterSaveOptions = _.assign({}, options, {
              permissions: false,
              workflowMissingLocalesLive: self.apos.argv.live ? true : 'liveOnly'
            });
            return self.docAfterSave(req, doc, afterSaveOptions, callback);
          }, callback);
        }
      }

    }