Is Keeper's Inheritance feature third-party independent?

Yes. Keeper's Inheritance feature is third-party independent (including Keeper itself!).

You can setup your inheritance plan according to your preferences. Keeper only provides tools and templates that act as guardrails to protect your assets.  

If you plan to use the Inheritance Key: You are welcome and encouraged to download the seed words of the Inheritance Key, thus ensuring that you are not dependent on, or restricted by Keeper (or Keeper's existence). Also, when you use the Inheritance Key, the total number of keys, expands, not the minimum. Thus you have more options to use the keys in your multisig.