So Lost...understanding what you can/cannot do w/ Layers

Hello!

Going to ask my question in bullet points, to simulate where I hit a wall...and not sure what to do or where I went wrong.

Adobe Photoshop CC 2019 / Mac 10.14.3 / 8 bit image / TIFF

My workflow for retouching a beauty image:

1.  Open Image

2.  Duplicate Image - On duplicate image I go to Filters, Liquify and make adjustments

3.  Duplicate #2 above and Crop Image (now I can see how I would like the final image to look and starting cleaning)

4.  Next I add a Blank Layer and clean up the skin using healing tool.

5.  Next on new layers I perform Frequency Separation and continue to clean the skin.

6.  Next on new layers I perform Dodge & Burn and continue to clean the skin

Enter the PROBLEM:

Oh oh, I forgot to Liquify the models eyes in Step #2 above...when I go back to this layer, I cannot do anything???

I cannot use my Brush to add eyelashes

I cannot use my Stamp Tool or Healing Brush to clean more of the skin.

The only way to move forward is to create a Stamp Layer to get back to a Background Layer Image...and start from there.

Problem... isn't this Destructive?  I'm now committing to the above 1-6 steps without an option to either delete or modify what I've done.

Will somebody please explain what you can and cannot do with Layers ...btw I am not using Smart Object layers...not sure why?