Are granted permissions layered and can they be subsequently peeled off?

Yes. For example, perform the following sequence:

  1. Grant user-level permission
  2. Grant channel level permission (this effectively overrides the user-level grant opening the channel up to anyone)
  3. Revoke channel level permission (removes the channel-level grant, but the user-level grant is still in place)

The outcome in this situation is that the user-level permission would prevail.