Creating a Playback Template fails

From what I understand, to create a Playback template, first I need to save all the Endpoints it contains (one for each VST single or Multi). Then, in the Playback Template Editor, add a couple Autos to catch any instruments not included by my custom additions, then manually add my new Endpoints, move em up so they have priority, then fill out the Family and Instrument Overrides… so far so good, right?
I did that and it seemed to work OK, but next use I wanted to change a couple- so I added them to the Project. OK so far, so good. Then, wanting to save them to the Playback Template, I opened the Playback Template editor, chose my previously saved Playback Template and saw the Endpoints already in there, prioritized, but without the Overrides showing. So, I added them, clicked “Apply,” and my new Endpoints disappeared from the project and the prior incomplete state of the Playback Template reverted itself onto the project. ??
OK. What is the procedure? I watched the video and followed the directions.
Is it possible to add Endpoints to an existing Playback Template?
Why would updating a Template cause your changes to disappear from the project?
Do I have to have the Playback Template open while the document wherein it was created is open? (I thought I heard that in the video, but didn’t do it, rather expecting or hoping that you could edit Playback Templates as independent entities…?)

Is it possible to create a Playback Template that is, in effect, a snapshot of everything currently loaded in the project? I’d like to be able to change or update the Playback Template without voiding my previous work.

Nobody?

One thing I am finding is that changes I make to a playback template will only reflect if the affected instrument is added to the project again. The overrides appear to only work on fresh instances of an instrument… that seems it might be core to this issue. In effect, I have to change the template before adding any instruments to the score. Please correct me if this is incorrect.

I have yet to have success creating an endpoint using a non-Dorico library for Drum Set.
It would be handy if I could just import saved Endpoints to a new project in Play mode without having to work through the whole Playback Template infrastructure - and if it would be possible to save and re-use a drumset VST endpoint, or make it work in a Playback Template.

Have a look at the Addictive Drums 2 template/endpoint and sample project in this thread: