Gift giving is not a problem, faculty smile when they see it.
But really, I'm a student dentist and all I want from my patients is for them to come on time and be happy to be there. Sometimes that is too much to ask, but atleast don't b@#$$ in the dental chair when I'm trying to help your disease-riden mouth. The worst are the patients who haven't been to the dentist in a decade, and think their teeth were okay, and find out they need 6 cavities filled. And it takes about 1 visit for each cavity, if they are large. 12 hours of the patient's time for problems they never had before they came to the dentist. Understandable, but I try very hard to educate them, still they are not appreciative. Appreciation is a gift in itself. Sounds like you are appreciative, so I wouldn't say a gift is necessary.
I'm curious what this student dentist was able to do for you.
Also, yes different dentists have different opinions on lesions. Caries are unpredictable. I saw a 14 y.o. the other day and he had no cavities except one 14 that blew up and had a pinpoint exposure while we were excavating the day. He was young, and that tooth was asymptomatic so it ended up being a vital pulpal therapy (referred to endo), but really you wouldn't expect such a cavity when no other tooth had even a stained pit - it was so wierd. Some dentists would probably say do a root canal on that tooth, and forget the vital pulp therapy. We won't know who is correct. It'll take a year to figure out if the vital pulp therapy worked. There are definitely different opinions in dentistry.