Integration test per layer is a good practice?

I have an application that use spring-mvc, basically we have a presentation layer (controllers), service layer (business units, helpers), integration layer and data access layer(jdbc/jpa repositories), we want to ensure using testing that future addition to the code won't break nothing that was previously working, to do this we are using unit testing(mockito) and integration testing (spring-test,spring-test-mvc).

Unit testing is made per class/component, basically we tried to have a good coverage for the incoming inputs and possible flows within these components and this action is working fine, not have doubts here as unit test is about ensure the units works as expected.

Integration test is different story and very debatable one, as for now we are using sometimes the same scenarios we use to design our unit testing but having the entire system available using real platform and so on, but I have doubts about the best practices here.

1. As we have a controller, service, data layer one approach is made an IT per layer, example we have UserService class, we will have UserServiceTest which will be the Unit test and UserServiceIT, but maintainability is not ideal, I feel sometimes we repeat the same test scenario but now using the real system. Does this practice really make sense or in which scenarios this makes sense ?. If we already have 100% test coverage in the class with unit testing why we need IT for this one, seems that we have this only to ensure real component is going to start-up ?, Make sense to have all the same scenarios or which is a good criteria to decide?

2. Other approach is just go with the most important test cases via integration test but just from the controller layer, which it means invoke the REST services and verify the JSON output. This is enough ?, we don't need to verify more things in the others layers ?. I know calling the real REST api will use underneath all the layers (controller, service, dao) but this is enough ? Some consideration you will say here?

3. If we have a helper class I don't think make sense to have unit and IT, as most of the method as there for only one purpose I think unit testing will be enough here, does you think the same?.

4. Some classes in the data layer could use Criteria API, QueryDSL for those I go using IT as make the unit testing in some cases is extremely difficult, this is a valid justification?

I am trying to get the best way, tips and practices that makes the task to ensure the system integrity a real and valuable process keeping in mind the maintainability of them.