1) How many credits compose a typical full post-bac?
That depends entirely on which/how many prereqs you have to take, as well as the credit values assigned to those courses by the postbacc institution. I earned 48 credits at my postbacc program, taking all the prereqs from scratch, plus precalc (3 credits) and an optional bio course (2 credits). So I guess the prereqs were worth 43 credits.
These were the courses my school required:
1 yr calc or calc + stats (6 credits)
1 yr inorganic chem + 3-credit lab (10)
1 yr physics + labs (8)
1 yr organic chem + 3-cr lab (10)
1 yr bio + 3-cr lab (9)
This was done over 2 years. If you had a given prereq class already, they expected you to take another upper-level course to fill your schedule. (Full load was considered 4 classes/semester in year 1, 3 in year 2).
2) How do most medical schools assess those additional credits? Are the grades from the post-bac classes simply added to your preexisting credits and the GPA averaged? Or is there some less discernable formula?
Postbacc classes count in your undergrad GPA [uGPA], because they are undergrad courses. AMCAS (the med school application clearinghouse) computes both a uGPA and a postbacc GPA for you, so schools can see separately how you did in the postbacc program/courses.
Do certain schools emphasize more recent (post-bac) coursework over your earlier grades? Or vice-versa?
Again, this depends heavily on your individual circumstances, mainly how long it's been since you graduated from college. AMCAS includes both ancient and recent grades in your uGPA (a practice I personally consider ludicrous, but I don't make the rules), and med schools tend to be rather robotically fixated on that uGPA, even more than the BCPM [science GPA]. However, if your undergrad grades are really old--mine are over 20 years old--schools will look more closely at your postbacc GPA, although they never completely ignore the uGPA. In your case it shouldn't really matter, since your uGPA is already good.
4) Is there any additional known way to raise one's GPA, beyond the post-bac programs? Say, non-science post-bac work at your alma mater, prior to formal post-bac science program? If so, are such efforts regarded as transparent by top medical schools, or is it a useful exercise?
Keep in mind that "post-bacc" merely means undergrad classes that you take after you have already earned a BA (baccalaureate) degree. You can do a formal program, or not--it's up to you, and med schools don't seem to care. But if you are going to take postbacc classes, I'd stick to science classes only (unless you're missing a required English class or something of that nature). You don't want to be seen as padding your GPA, and you do want to demonstrate your prowess in science as much as possible.
Just make sure you take UNDERGRAD courses, though, or they won't count toward the all-important uGPA. As you may have already read in other threads, graduate work (even PhD's in the sciences) is not given much weight by med schools, no matter how meaty it is or how high your grades are. According to PhD's who've been admitted to med school, their degrees are considered no more than a "nice EC" by admissions committees.