[HaseBeceDeemy]

It would be appreciated if a Thai person could answer this as only they will understand the nuances:
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Question 1:
I give a small gift to someone like a hotel receptionist and she does a "wai" of thanks. Should I return the "wai"? Or is just a nod, smile and "Mai ben rai" okay? Or maybe say "Indee khog Pom Krap"?

Question 2:
I have a Thai teacher who is much younger than I am. I am also an "acharn" but she has never been my student:
At the beginning and end of each class should I "wai" her first as my teacher?

If I were to run into her in two years after class is over (and again, I am about 20 years older than her) should I "wai" her first again: In other words, once a teacher, always my teacher?

Question 3: How sensitive are Thais to this "wai" custom meaning if a farang gets in wrong (does not wai a teacher first etc) will they be offended or will they just let it slide as another "farang mistake"?
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Thank you!