1)  say "_one of my strong points_".  (you do have others). Avoid using "forte", because either the interviewer won't know it, and will see you as talking too fancy, or she will know the word, but will "know" it's pronounced For-TAY, so when you pronounce it FORT, she'll think you got it wrong  (or vice versa!)

"Forte" is from French; the E was originally silent.  But in America, it got confused with the related Italian word, used in musical notation, spelled exactly the same way, but with the E _pronounced_!

https://en.m.wiktionary.org/wiki/forte#/search

2) I think it's OK to use both, as both strength and stamina are required as a flight attendant.

3) To keep it short, say "[my experience] in Cheerleading and Track". 

> Don't bother to call it "Track and Field". **Track** is enough.(AmE)

3-2) Yes, "built and developed" is redundant.  Use "developed" (you already had some strength and stamina; now you have more).

4) Say "...[my strength and energy] will help me to (easily) adapt to working frequent long-distance flights"  [and "varying schedules"—which they _will_ask you to do!].  

> Your strength might also help you save lives, in an emergency !
(But don't say your energy is "amazing"—that's bragging!)
https://en.m.wiktionary.org/wiki/brag


5)  a) Don't talk about donating blood; it might _gross  out_ the interviewer. 

http://idioms.thefreedictionary.com/gross+out

> You can  make your point about strength and stamina without mentioning blood. If you want to convey that your heart and lungs are strong and resilient, describe your athletic experience as a form of  "aerobics" and/or "cardio" exercise, which made you "fit and healthy" and "ready for any physical challenges the job might present", or some such phrasing.

  b) They may be less interested in your physical stamina than in your attitude and "people skills" (communication and cooperation), so emphasize these as well. Fortunately, you can honestly say that you _developed both these skills as well_,  in Track and Cheerleading.

 >  b-1) As a cheerleader, you had to maintain (and show) a positive, cheerful attitude as well as learning teamwork. 


>   b-2) You probably spent time traveling in buses with your cheerleading squad and Track team, so you learned "how to _get along with teammates_"  [for extended periods] "**in tight quarters**", (which is _quite important for flight attendants_.)

I know this advice goes far beyond grammar; but I hope it helps, and I wish you the best of luck and confidence at your interview. You'll do great—Happy flying!