After spending most of the afternoon scouring these texts, and working through indices, where available, I can report that these classical authors had absolutely nothing to say about what happens for the 24 days that Mercury goes retrograde each of three times per year. That's right. Nothing. Certainly, they have a lot to say about interpreting how a retrograde Mercury in a chart will affect the outcome of whatever that chart is about. But they don't seem to make any attempt at classifying or qualifying a moving period of time under this influence. It seems that until very recently in the history of astrology, no one was really interested in interpreting the meanings of astrological phenomena outside the context of an actual chart. The chart in question often covered a period of time (e.g. the Aries ingress chart was often used to predict world/mundane events for the coming year); but that chart was then moved through time on a symbolic level as well as watching how the planets continued to move. But these planetary movements were always related back to that chart for the purposes of the interpretation at hand. Nowhere do they seem to have said anything like "when Mercury goes retrograde, the world tends to go wonky."
(If anyone knows of any classical sources that address this issue I would appreciate the reference in order to do more research.)
Does this mean that Mercury retrograde has no effects? I don't think so; I have witnessed some of them myself. But I think that far too many negative events are attributed to Mercury than he deserves. As a matter of fact, far too many things are attributed to Mercury than should be.
Some speculation, based on classical tradition
First, let's consider that even some classical authors thought that a retrograde Mercury could be improved by being connected to the benefic planets in a chart (Jupiter, Venus, sometimes Moon). And if we look at this particular Mercury retro, we see that Mercury is in Aries, where he is middling in terms of dignity (that is, he's neither well dignified nor ill-dignified). Until April 13 (EDT) Mercury is heading right at a well-dignified Jupiter in Aries. The Moon makes a few contacts along the way as well. But Jupiter is a benefic, and does well in the fire signs. This would seem to indicate that the negative affects of Mercury will be much lessened until at least the 13th. However, one must also consider that during much of this period, Mercury is combust - conjunct the Sun. Classically this is a great debility, not because it makes Mercury do "bad" things, but rather because the Sun burns up any power that Mercury may have. So while Mercury itself may be fairly ineffective during this time, it may be helped a bit by Jupiter.
For the next (and final) 10 days of the retrograde, Mercury is in opposition to Saturn (a malefic) in Libra. Again, this will be interrupted off-and-on by aspects from the Moon. But these last 10 days look as if they will be the worst part of the retrograde due to the Saturn influence.
Of course, as the title of this section implies, this is speculation on my part, but based on what is written in the classical authors on this subject, rather than a mere exercise in free-association of ideas.
I have seen recently some pretty fantastic (as in "fantasy-based") lists of what Mercury rules, with no citations. Lee Lehman, in her Book of Rulerships, scoured nine major classical astrological authors, from Ptolemy through the 17th century, and compiled lists of what rules what based upon their texts. Using her book, here is a partial list of what Mercury rules or is associated with:
Accountants, Affairs, Arithmeticians, Astrology, Attorneys, Bankers, Bargains, Birds, Business(men), Buyers, Cheating, Clerks, Coins, Commerce, Contracts, Counterfeits, Curiosity, Dexterity, Divination, Educated People, Fairs (flea markets, bazaars), Hurricanes, Imagination, Intellect, Journeys, Logic, Mathematics, Memory, Negotiations, Poetry, Printers (the people), Rationality, Reason, Schools, Scribes, Secretaries, Servants (non-contractor employees today), Speech, Teachers, Tradesmen, Writing.We could extend this list logically to modern areas that are developments and newer versions of older things. For example, Printers refers to the people but by extension to modern printing machines (since Mercury also rules inventions and writing). Given Mercury's rulership of Writing, Logic, Inventions (etc.), computers are clearly a Mercury-ruled area.
(Note that "Communications" is not an area that is listed among the nine classical authors that Lehman surveys, yet somehow this is the first thing that we often think of when we think of Mercury retrograde. A few authors gave rulership of Messengers to Mercury, but not universally; others gave it to the Moon, and "Messages" was given to the Moon, or to the 3rd or 5th houses in a chart [depending on what kind of message it is]. This points out the modern astrological tendency to invent rulerships without grounding them in the Tradition.)
Now while the classical authors don't delineate Mercury retrograde specifically (other than to delineate Mercury debilitated by any means), Bonatti (and others) refer to retrogrades in terms of complications, distresses, defects, destruction, disagreement, loss, lack, reversals, etc. So a logical conclusion might be that there would be setbacks of these types in these Mercury-ruled areas. But not automatically; the "right" (wrong?) connections must be made.
So I'll repeat what has at times almost seemed a mantra: calm down, people. You will survive this retrograde as you have survived all the ones before it. Leave yourself some extra time and be a little extra careful, and you'll be fine. And when this retrograde is over, and life still isn't perfect, I hope that we all realize that we've been making Mercury our scapegoat/whipping-boy for far too long; there are other things going on in the sky, and they're worth some attention too.