I not really qualified to answer your questions, but here are some guess-timates:
>So I may get a better sense of this, what kind of percentages for shrinkage might typically occur in earthenware for:
>1) the first bisque firing (6% or so, for example?) and then...
>2) the second firing after glazing (an additional say 3% for a cumulative total of 9%, for example?) and...
AFAIK you're in the right ballpark, there is also shrinkage from forming to being air-dry. [This is low for dry-pressed ware.]
>3) are the temperatures and time the same for both stages of firing?
>This assumes firing to maturity in the end.
No and yes-ish. The bisque firing takes the pot very slowly through some risky parts of the firing (e.g. getting rid of chemically
combined water, burning out organics, etc.). The end temperature of the bisque is probably chosen so that the absorbency
of the bisque pot is optimal for dip-glazing. The "glaze" firing can go rapidly through the lower temperatures, but then needs
to reach sufficient temperature -- for sufficient time -- to mature the pot "as desired". AFAIK this ends up with the two firings
taking comparable times. But actual firing schedules depend on the body material, the kiln, and to some extent the whims of
the potter and his/her kiln-controller.
>It is hoped that tolerances can be reliably kept within +/- 1% after shrinkage, after the entire process has been tested.
This is an area where you do need a consultant in technical ceramics. If resonant cavities are involved remember 1% linear
is 3% volume.
You may find acquiring a general reference work, such as The Potter's Dictionary of Materials and Techniques helpful.
Two other surface treatments you might like to consider are Parian and salt-glaze.
Parian is a sort of self-glazing porcelain casting body. So named because of its visual similarity to Parian marble.
Salt-glazing is a specialist technique that gives a very thin surface film of glaze, sometimes pitted in
an orange-peel effect, most usually associated with historic German beer steins and sewer-pipes. Its
a specialist process, which gives a covering that is often attractive and usually very dirt-resistant [and
you do rather want dirt resistance in a sewer pipe].
... perhaps in your case the less obtrusive effects would be appropriate