Corrid confusion... Should I use it to make sure or not?

DE will not kill the cocci protozoa but I doubt it will kill your chickens, either. A bunch of people add it to feed as a wormer. (It doesn't work for that, either.) Actually it is sometimes added to commercial feed in small amounts.

DE is one of those things that there are some strong opinions on. You might read some of the threads about it that are linked on the FAQ page. One fact, not opinion, is that it is quite irritating to lungs, yours and the chickens'. I do use it, sparingly, in the litter to help control odors. Hopefully it also controls mites/lice, but I won't make that claim. I have also used it in the garden as a "harmless" (not true) insecticide. It does do a good job of controlling worms/caterpillars, like tomato hormworms and corn earworms (if you get it inside the ear.)

One teaspoon is 5 cc or 5 ml, so 9.5 ml or cc is just under 2 teaspoons.

You have said that you bought medicated feed. Medicated with what? Usually it is amprolium, which is the same thing as Corid, only a lower dose, intended to help them build a natural immunity to cocci. But you can also find feeds that are medicated with an antibiotic. At one point, TSC sold an unmedicated feed and had a sign over it "reminding" people to buy some antibiotic to give along with it. It is well known now that giving antibiotics as a preventive, or when you do not know that the particular bacteria is present, is not a good idea at all. Amprolium is not an antibiotic. I do use it for new chicks.

Chicks shed intestinal lining when they are quite young, which looks like blood, and probably has a little blood in it, but it is a normal process. Trick is to differentiate between this and real bloody poops, which, as you know, mean cocci. Since you had one die, you may indeed be dealing with an infestation of cocci.