How do I get Matrix elements to copy their content across language Locales?
Ok, in some conversation with Brandon which leaned this direction, I realized a different direction to temporarily solving most of this problem.

It turns out that a root of the issues is that marking a Matrix as translatable causes the base problem here, that the set of elements in an original language isn't copying at all into other Locales, as would be expected on first Save as all other kinds of elements do.

If you don't mark the Matrix as translatable, two good things happen:


All Matrix elements now do get copied over, so that you don't have to recreate the Matrix in its individual form for each Entry and language.
Text items marked as translatable do have their original text copied over, so that you have a starting situation for language translation


It's true that Assets marked as translatable show up empty rather than with their original content, but at least they are there.

We also don't really know what marking a Matrix as translatable should mean. I suspect it should mean that all contained elements are marked translatable, but we shall see.

The Craft crew is well aware of issues here, and plans a careful improvement of the whole area, which as complex isn't at the top of lists yet. 

This little detail should get most of us through until that happens, and given we set up the non-translatability of Matrix elements, should make extra steps explainable to customer end users.