JsonPath approach

file.java

// <input name="applicant.employments[2].employerName" type="text" />

// employer.name
// employer.address.zip
// employer.address.street

class Update {
  UpdatePath path;
  Value value;
}

class UpdatePath {
  // { applicant, pet's names, 1 }
  // e.g. "applicant.dependents[1].employers[2].address.zip"
  // becomes
  // { applicant, dependents, 1, employers, 2, address, zip }
  List<String> path;

  List<String> getParentPath() {
    return path.subList(0, path.size() - 1);
  }

  String getPropertyName() {
    return Iterables.last(path);
  }
}

class UpdateResult {
  boolean success;
  Map<String, String> validationMessages;

  UpdateResult(success, validationMessages) {
    // etc.
  }

  boolean isSuccess() {
    return success;
  }
}

class UpdateStrategyFactory {
  UpdateStrategy strategyFor(ApplicationVersion version, String path) {
    // get the specific application version out of the questions table
    // use that to figure out which type this path corresponds to
    // get the update strategy for that type
  }
}

// Processing a form submission
// *************************************************************************

DynamicForm requestData = formFactory.form().bindFromRequest(request);

// form submissions include a list of updates
List<Update> updates = requestData
                       .getFields()
                       .stream()
                       .map(field -> new Update(field.getName(), field.getValue()));

// updates are applied
Applicant applicant = Ebean.find(Applicant.class)
    .email.equalTo("rob@foo.bar")
    .findOne();

DocumentContext applicantData = applicant.getObject();
List<UpdateStrategy> strategies = new ArrayList();

updates.forEach(update -> {
      String parentPath = update.getParentJsonPath();
      JsonNode node = applicantData.get(parentJsonPath);

      if (node == null) {
        // gets the JSON object for this path. if the path is
        // invalid, it throws
        node = answerNodeFactory.getNodeForPath(parentPath);
      }

      UpdateStrategy strategy = updateStrategyFactory
          .strategyFor(applicant.getApplicationVersion(), parentPath);

      strategy.setTargetNode(node);
      strategy.setUpdate(update);

      strategy.updateTarget();
    });

// run validations on all "dirty" objects
Result result = ValidatorFactory.runAllFor(strategies);

// if validations pass, new state is persisted
if (result.isSuccess()) {
  applicant.save();
  return ok(...);
} else {
  // or something like this, messages will need to be mapped to form input elements
  return badRequest(formState, result.errorMessages);
}