Thanks for a wonderfully detailed reply! My responses to your questions, and results of trying the suggested actions, are below.
When you refer to child components, do you just mean components, or do you mean ‘child actor components’? Totally different thing.
I’m sorry, I meant “child actor components”.
Experimentally, I tried adding a static mesh component as a child and attached the appropriate asset file to it. This works as you indicated, with respect to being instantly updated. (Hooray!) If I double click it as shown in your first image, it brings up the standard mesh editor but not the new Modeling mode from the plugin. Here’s what I get:
The one I want is this:
So far I’ve only found this to be available from the viewports in the main window, which is okay – but it would be great if that could be invoked within the BP editor. OTOH, I realize this plugin is pre-release, so I’m not complaining – just checking to see if I overlooked an easy way to do this. 
The world hierarchy question I asked may not have been clearly stated, so let me illustrate with some screen shots (which is what I should have done the first time – sorry!).
This is the area I think you were explaining how to close:
I hadn’t noticed the resizing option for that, so thanks for pointing this out! That said, the area that I want to influence is here:
As you can see, there are a lot of those “static mesh actor” entities, and AFAICT the only thing you can do with them is toggle their visibility. A left-click on any of these simply selects the parent. So I’d like to have this part of the world outliner collapsed by default for each BP.