I tried moving the Unity Editor Extensions to a folder named 3rdPartyAssets to fit with our project structure (which is possible according to https://github.com/PlayFab/UnityEditorExtensions), which led to an exception:
NullReferenceException at PlayFabEditorExtensions/Editor/PlayFabEditor.cs:151
The problem appears to be that PlayFabEditorHelper.GetUiStyle() has
eachPath.Substring(eachPath.LastIndexOf("Assets"));
which incorrectly matched our 3rdPartyAssets path.
I edited that, and installed & relocated the SDK, but then the code to detect the relocation of EditorExtensions failed, because a copy of the unity package was stored in the default location, and PlayFabEditorHelper interprets the presence of files in the default location as evidence that EditorExtensions hasn't been moved.
Finally, the EdExPath in the PlayFabEditorPrefsSO scriptable object contained the absolute path of the folder, which obviously was incorrect on our other users' machines,causing new default files to be generated. I modified this to be a relative path.