Once you are able to load the detection checkpoints into their respective model class I think it should be possible to access the layers just by their indices.
OD API models may often seem to be a bit opaque. I would also suggest taking a look at the Model Garden object detection models since they seem to be more transparent and accessible with regards to the interpretability aspects:

  
      

      github.com
  

  
    

tensorflow/models 44

  master/official/vision/detection

  Models and examples built with TensorFlow. Contribute to tensorflow/models development by creating an account on GitHub.