If I skip the "make a scene view" step, still get the effects not changing (though it seems like in this case get button press feedback but not GUI swap out). I also have to set the master volume after loading the MRB.

Some more testing: if I save just as a MRML file, close the scene and reload it: same issue in the Editor. Suspect that it's sometjing about making the default master scene view on save of mrml or mrb.

A few things are contributing to this bug:

Singleton tags are not written to XML. When a scene is read in with a scripted module node for the editor, it only has ModuleName set (Editor), the singleton tag (Editor) wasn't saved.

So on importing from the MRML file on disk, if you're in the Editor module, there is a singleton editor parameter node in the scene, but the incoming scripted module node isn't detected as a singleton so both are present in the scene.

In EditUtil.py _findParameterNodeInScene, it looks through the scene only checking for scripted module nodes that have the ModuleName set to Editor, and after loading a new scene, the new one is found first and used, even though it's not tagged as a singleton.

I did quick test to see if I could fix it inside the Editor rather than having to change the vtkMRMLNode Read/Write XML to read and print out the singleton tags, and expanding the test in _findParameterNodeInScene seems to have fixed the "not able to switch tools" problem by making sure it uses the original singleton editor parameter node rather than the one that was in the scene on disk:
if n.GetModuleName() == "Editor" and n.GetSingletonTag() == "Editor":

But in a larger sense, not having the singleton tags in the XML file means that any (most?) singleton nodes aren't going to get updated properly on loading in a new MRML scene.

I did some quick testing and I agree with your assessment. It seems we can fix the Editor now by adding the check for the SingletonTag as you suggest.

Longer term I don't see any problems (only benefits) from including the SingletonTag in the XML. Not sure why it wasn't included in the first case.

Okay, will make that quick fix.
JC and I talked about it on the dev hangout and he suggested asking the dev list for feedback to see if there are any historical reasons for leaving them out of MRML. Is Alex still watching the dev list? The original commit that JC found was from him.

Editor effects work on scene load in svn 24528:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=24528
and git:
https://github.com/Slicer/Slicer/commit/b1bb1b7c30bd76258414b2c6ba527f80f583c49a

Confirmed - nice!

Pull request for serialising the singleton tags:
https://github.com/Slicer/Slicer/pull/357

Serialising singleton nodes integrated in:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=24582
https://github.com/Slicer/Slicer/commit/808c8e68438c8fff6e77f4886b67facb77659a9b

Updated two tests with new baseline mrml files:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=24585
http://viewvc.slicer.org/viewvc.cgi/Slicer3?view=revision&revision=17107