The solution was included as an edit and addendum to my original post.
If you dimension and set a PowerPoint presentation it will reside in memory, but not displayed on screen (insofar as I can understand).
The presentation must be initialized to be displayed. This is why some solutions suggest saving and closing the presentation, then reopening it to see it. But reinitializing in that way is not required. You can simply display the presentation in a new window.
e.g:
Code:
Set pptPres = pptApp.Presentations.Add(msoFalse) ' Add the presentation hidden
pptPres.NewWindow 'Present that presentation in a new window (initialise it)
Note that if you set the presentation visible, it will already be initialized into the new window, but then you'll be stuck watching the presentation build.