(1) Is there a textbook?

Likely several, look for high school texts in second-hand
shops, I picked one up not too long ago and found it
both educational and entertaining. The one I found is
"Machine Tool Practices", ISBN 0-13-541848-8, I paid
$1.98 at Half Price Books in FW, TX. Your results may
vary.

Does it come with a set of tests that I can
administer to myself to determine whether I should be
allowed to move on to the milling machine or whether
I should be left back and have to take File 101 over
again?

In the case of the above, yes.

Do I need just one file to get what I need out of the
course or do I need to get a set of files with various
properties and characteristics?

While one would suffice to start the course, several will
be of use long after you own said milling machine. To wit:
Medium and coarse flat files (mill or mill *******),
a medium and coarse square, a triangular, a coarse round,
a half-round, and a chainsaw file (no particular size,
since you're using it for general stuff rather than a saw
chain).

How much should I expect
to pay for the lot of them?

New, I have no idea. I buy used, and usually pay a buck
each, with this caveat: if you run your thumb over it
and it doesn't want to grab, it's dull, and not worth the
buck. If you look at it and see a shine from the top of
every tooth, it's dull, too. Buy at least one or two new
Nicholson (non-cheapistani) files so you have a good idea
what a sharp file should work like.

(3) Presumably, I also need something to use the file(s)
on. What might that be?
Perhaps an assortment of different pieces of metal in
different conditions requiring treatment with the file(s).

AKA, scrap iron. Plain old mild steel works well for this.
In a pinch, you can even buy it at hardware stores in
flats, angles and rounds.

Where do I get them?

Junkyard, side of the road, tagsales, dumpster diving.
It's a hobby in and of itself.

(4) I doubt that there is a shop that sells metal with
burrs on it to people who need practice removing burrs
with a file, so I probably need to learn how to take a
piece of metal and modify it in some way so that I can
practice filing it. [...] probably the first thing
the course should do is explain how to ruin metal in
fairly specific ways.

The easiest way is to set out to make something, be it
useful or decorative, out of said scrap, which will
nearly never be of the proper shape for the intended use.

First step is to mark out with a scribe or marker
(depending on how accurate your final piece needs to be)
the desired end shape.

The next step is to get it close to that shape by
applying a coarse brute force method. A hacksaw
is typical, though people have been known to resort
to torches and bandsaws, or even several of the
above. The idea here is to just miss the line you
will eventually file to.

All of these processes usually leave a coarse surface
more than adequate for cleaning up with a file.

(5) What tools would be ok to use along with files?
chisel, drill, files, hacksaw.

All of these will be of use, but particularly the hacksaw.
Buy a good one with a high tension adjustment and several
different tooth counts of US made name brand blades. The
difference between this and a cheap saw frame holding
cheapistani blades is an education in itself.

So, the question (2) is naturally widened to include
these other tools.

I'd pay about $20 for a good hacksaw frame, and maybe that
much again on several packages of good quality blades.

(6) Once the knowledge is imparted of how to ruin a
piece of metal in order to leave it in a predictable
condition requiring a particular technique to correct,
it then becomes possible to focus on those particular
corrective techniques. That might be considered the
proper content of the course.

Indeed, and this is where said tech school text will be
of particualr help, but you are also more than welcome
to run particular questions past the accumulated wisdom
of the group.

As far as the matter of competence in the
use of files is concerned, that would probably
be an adequate preparation for moving on to the
milling machine.

I will merely note that while the textbook I mention
is nominally about machines tools, they start by
teaching you how to file. A review of the google
archives searching for the words: apprentice, file,
metalworking should turn up a lot of relevant
discussion, particularly if you add "Bastow" to the
list. A bunch of this may also be on yarchive.net/metal
as well.

(9) I will merely mention the pedagogical issue of
whether, in teaching myself File 101, I should cover
topics [...] which I am unlikely to actually need in
practice.

Unlikely. In fact one of the best ways to learn is
to actually set out to make something practical.

I vaguely recall that I have in the past read
discussions about using only handtools, and that
there is actually a school of thought devoted to
this high art. I'm not sure whether I read it here
or on a woodworking group.

AKA, Neanderthals. Less common in metalworking, due to
the amount of effort needed to remove large amounts of
metal, though they do exist. They tend to work on smaller,
high value projects, and will frequently respond to the
title of "jeweler".

HTH, and I think your approach to the subject is
commendable, I like your thoroughness, and suspect
you'll do well with it.

Later,
--Glenn Lyford