(The "Ma'ma" in my post should have been Ma'am. Sorry for that. )
Merely a typo, no problem.
"Dear Sir, Dear Madam" is more used in writing than in speech. But if it's used, it's not unacceptable.
I wouldn't use it if I didn't know them,
an individual in person
when applied to a single person in face.
, or a single person face to face
* "Esteemed XXX" (Esteemed Examiners, in my case) is appropriate and is helpful to relieve the headache of considering the sex, age or marriage status. But it might be overformaml when applied to a single person in face.
* If I do know their names and title(seldom in such an exam's case - it's intended that you don't know them.), I can call them in the form of "Professor John", "Dr. Jane", etc.
* Eye contacts and expressions, etc, are part of the presentation and integrated with the words used in speech. Together they will decide whether the address make others feel respected and comfortable or not.
Again I agree. And, by the way this is very well worded.
* For a single person, I might be able to directly look at him/her and greet. In this case, I call a gentleman "Sir", and call a lady "Madam". So I would say, "Good Morning, Sir. I'm happy to be here to ...", or "Good Morning, Madam. I'm happy to be here to ...". Will this be ok in this single-person case?
Yes, you could also use it if there are two or three people by greeting the lady and gentleman separately (Good morning, Sir. Good morning, Madam. I am happy...) or, if there are more than one of each gender replace "Sir" with "Gentlemen" and "Madam/Ma'am" with "Ladies."
* "To whom it may concerns" can do well, but might be too formal or distant in talk to my understanding.
Don't use this in a face to face setting. It is only used in writing and when you are not sure who will receive the letter. For example if you are writing to the complaints department of a large store and don't know the name or title of the person who will receive and, hopefully, deal with the issue.
You have written in the past tense. Is the address over and if so how did it go?