Hi Paul,
Some good questions there, and ones that can get pretty involved. I'll go for short answers, which will likely lead to more questions
Yes, they did it with overlapping inks. Most color printing is done that way even today. T206 is odd in that it appears that at least one dark color was done early (brown or black) typically black is done last.
Yes, one plate for each color. there are only a handful of high production processes that can do multiple colors from one plate and the process used for most cards isn't one of them.
For T206s the printing process is lithography which is done from plates of various sizes. The plates were originally limestone, with coated aluminum coming along right about 1910. I believe the T206s were printed from stones.
With this process you can do all one card or a variety. There are T206s showing parts of two different cards so we know they printed multiple cards on a sheet. The place I worked had presses capable of printing on envelopes or on paper up to 36" wide. Larger presses were available. It's a matter of some research just how large a T206 sheet would have been, and how many different cards would have been on it.
Once the plates have been made it's difficult to make some changes and easy to make others. Moving or replacing a player would be very difficult and the easiest way would be to make a new set of plates.
Demmitt and O Hara - The plates would have been costly, but not extremely costly.
The basics of the process is a medium that will hold water is coated with an oily substance where you want it to print. The plate is then wet and while wet inked with an oily ink that stays only on the greasy bits.
To make a major change like Demmit or Ohara the plate would have to be cleaned in the one section and redone.
Fortunately for the printers they were using lots of colors, and team names on the uniform were often done in brown, as were the captions at the bottom. so they probably made a new brown plate.
That's complicated since those two are polar bear only. So it's more likely that the team names were changed on the original art and those corrected images were used on a sheet that included subjects in the 350-460 series.
Again, a subject for some research. Either scenario is a bit odd, but possible, the second being more likely.
So no moveable type was involved, but correcting the team names would have taken some effort.
Something like the Doyle would have been a much simpler correction, there's a special limestone crayon that can be used to fix stray lines or scratches in the plate, and the fix for something like having an unwanted "natl" would be to simply erase it on the plate. That doesn't even require removing the plate from the press.
Steve B