Eight hours to open gifts? I grew up in a family of nine, and even when we got old enough to take turns opening gifts (instead of tearing them open all at once), it never took longer than two hours, max. Is it the number of gifts or does each and every gift have to get passed around and exclaimed over? Does everyone have to bring all their gifts to one person's house and everyone opens them there, instead of opening immediate family gifts at home and then gathering together and opening the gifts extended relatives are exchanging?
I'd have an extremely hard time sitting there for 8 hours, most of that time spent watching other people open gifts. Especially if the kids were running amok elsewhere in the house.
Does DH's family have a "head of the family?" Someone who everyone else listens to? Can you or your DH talk easily with this person? Because what I'd do is try to get that person or persons on my side. I'd start by telling them that you and DH are trying very hard to de-commercialize the holidays for yourselves and your daughter. That you want her to experience Christmas in other ways than just getting presents. And then ask if they have any idea on how to help you achieve that on Christmas Day, where the entire focus is on opening gifts.
Or tell them that while you enjoy getting gifts, the past few years, Christmas Day has been all about the gift opening, and you and DH would like to have a chance to play with the kids and chat with everyone, instead of focusing so much on the gift opening. Could something be done to move the gift opening along a little faster, so that there is more free time for family togetherness and building snowmen and playing board games? Or could your little family unit join the rest of the family later in the day, so that you can do some special things with your daughter at home, and not affect the rest of the family's gift opening?
And you can definitely ask everyone to limit the gifts to your daughter. I think it's harder to limit gifts to yourself, for some reason. But you can definitely tell people to give only one gift per family unit to your daughter. You can also tell people that if she gets too many gifts on Christmas Day, you will be allowing her to open X number of gifts (X being a reasonable number) right then, and then giving her the rest of the gifts later. There will undoubtedly be flak about this, so you will have to stand firm.
My niece was overwhelmed by presents when she was little. My brother and SIL had to travel to have Christmas with the family, so they asked everyone before Christmas what we were giving Niece. They chose 4 or 5 gifts for her to open on Christmas morning, and saved the rest to be opened on the trip home. Yes, it was a little sad not getting to see her open my gift. But my brother had very good reasons for doing the gifts this way, and it kept Niece quiet and occupied on the plane, so we really couldn't complain.