Hi there! I will try to give my opinion about your topic so that our post becomes active. Sorry but I don't know much about this field
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1. Is research experience really required for admission?
It always help. Some universities value it more than others ( Harvard for example). However this field is a lot more related to research than most of the fields, so I believe it might count more. Just a guess.
I think it is not a competitive field. Research experience would be a plus, but I am sure they look at the whole package ( good scores, recommendation letters, SOP, etc)
2. Scope of this field
I believe this area seems to be a lot more related to research, after all you are learning and testing the different materials and technologies. Of course nothing stops you from doing clinical work. One might wonder how much this area increases your clinical skills tho!
I think your future would be more related to a faculty position. There is a faculty shortage in America so you would be in advantage if that was your focus, an academic career ( teaching/ researching). The reason for this lack of faculty is the fact that they make less money, so most dentists prefer a clinical career.
3. Any Univ accepting foreign trained dentists without GRE? Coz most of Univ sites I went thru have TOEFL and GRE as requirements, some even ask for NBDE I scores.
You better check the website of each school, that is where you will find more accurate details about requirements. I think most schools require the GRE for a masters degree, after all, they want to test especially your analytical writing/thinking and vocabulary since you will have to write a thesis.
There are tons of books that prepares you for that tho, just like TOEFL.
I have to say it was the hardest exam I have ever taken

When I applied for perio all the schools required that. The exception was for those who did not want to combine a certificate with the master's degree.
I would recommend you to post this topic on the residency forum. You might get more information there. And of course, visit the website of the schools you are interested in!
Good luck!!!