The AGI certificate requires two exams: FOI and also AGI (or whatever it's called). Does the Flight Instructor Airplane (FIA) exam give a credit for the AGI exam?
If you want your AGI and CFI, then you need to do both exams FIA and AGI, plus obviously the FOI. No crossing credit the AGI has some questions the FIA doesn't that pertain to helicopters, etc.
No. The AGI needs to be it's own exam. The only thing is that the Flight Instructor Certificate can be used to substitute the FOI exam requirement, since you are assumed to have already accomplished that part.
No. That's why they have a AGI test and a CFI(A) test. Notice that the AGI has no category or class limitations. There can be glider/helicopter/ balloon and now I guess powered lift questions on the test.
Not to discourage, I took both the tests and they were VERY similar except maybe 2 helicopter questions that threw me off; however still scored over a 90. The hardest part was having to pay the extra 90ish bucks to take the same test twice. Good luck!