You posed a lot of questions here, so I will offer what I can....
As Patrick stated, hearing a percentile would indicate that she did PennHip - the most scientific basis for grading the hips and honestly, more reliable than even the OFA. Before purchasing him however, you should have requested proof of all health certifications and had her explain them all to you. So if there was a Pennhip evaluation done, ask to see it.
Keep in mind however, that both parents may have been certified with clear hips, by either the OFA or Pennhip and this still does not guarantee that your pup will be free of it. Simply looking at the parents is not sufficient, and knowing what the grandparents also looked like, as well as all the offspring those grandparents produced (your pup's parents' littermates) is what is key. Good pedigree analysis, including a vertical evaluation is your best option for evaluating a pup's health prospects.
See this OFA article to understand more: http://www.offa.org/hovanart.pdf
You asked what you are entitled to if there is a problem - that all depends on your contract with the breeder. Good breeders provide contracts to protect the interests of the animal. Your contract outlines any health guarantee, and commitments by the breeder. My contract for example states that I will get money back for completing each health certification on my dog for example, and if there is a crippling problem, I get my money back. If you do not have a contract, then you are likely not entitled to much of anything. You made a direct, albeit blind purchase, with the breeder and it becomes "buyer beware" if you did not get a contract.
I took a look at your video - and honestly, there is no reason to panic. His gait is not good (but so are a LOT of dogs) or "correct" but that does not mean he has hip dysplasia necessarily. Keep in mind that he is not limping, so if his hips are not clear, they are not likely severely affected either. It looks like he can live a good quality life despite his gait, and may not have any issues for a long time.
Most giant breed dogs, and certainly many many newfies are dysplastic to some degree. There are also a host of issues that are possibilities affecting gait, so it could also be laxity in the hock (also a common issue in the breed) or just poor structure or angulation from the back causing poor gait. If it is a structure weakness, then it is what it is and your focus will be on management.
The important thing to do for him is ensure proper exercise and proper muscle development. I have a 6 month old rescue Bernese Mtn Dog here right now and he came out of a petstore cage completely down on his hocks, barely able to walk, with no muscle development whatsoever. After swimming therapy and proper exercise for the past month, this boy looks remarkably better, fully upright, walking normally and getting stronger. He has both a bad hip (dysplastic) and a bad elbow (dysplastic) but is looking quite good given his condition. We will focus on continuing his therapy and building strength, good supplements and a raw diet, and place him in a good home that will continue these things....he'll have a good quality of life. So just keep that in mind.....
If you are really concerned, and want to learn more, then you should get involved with your local Newf club - those are your best breed experts and will happily educate you on your dog - just ask the show folks for an evaluation - and seek a consult with a good orthopedic vet who can x-ray him and submit to OFA.