Oops, that title turned into a mess, but hopefully someone will correct it.

I forgot to ask whether there are any considerations for spaces before and after the en dash.

There are *conventions* -- not regulations. They also change over time, and with general style.

For example, the four-to-em space you mention with guillemots in French wouldn't be right if someone were setting ragged right and was using a font where four-to-em was a normal word space. Remember, you get a 4-to-em space with a spaceband value of 250 units (with a 1000 unit em).

Most newer fonts use a smaller nominal value than 250 units for the spaceband -- I've seen 190 units a fair bit, and that's smaller than a 5-to-em. If you use a hard 4-to-em between the guillemots and a word, the resulting space would be larger than the nominal word space when set with many fonts.

As to periods after initials -- it is a matter of style. Chicago style is to us a word space with initials, as in

G. E. M. Anscombe

Associated Press style is to not use any extra space

G.E.M. Anscombe

Etc. in other constructions.

So -- you get a lot of theories from typographers, but rarely the assumptions they are based on -- e.g., how big is a wordspace. Moreover, in the largest portion of the world of words, editorial style is the determining factor. Typographer's opinions are often marked "P.E."

In the case of an abbreviation such as Ph.D., the answer to the question of what kind of space to put after the period inside the abbreviation is usually "none", at least in the United States. In most editorial styles, for such abbreviations the periods are closed up or else omitted: "Ph.D." or "PhD", with the latter being the more common nowadays.