I don't know the physiology, but of all the dogs I have had, and that is many, they all have done this.
yes
dogs can never bark upside down because they can only bark when they are standing right side up!!!!!
because they might have allergies
Yes dogs can in fact sneeze. They do and it is kind of funny when they do , but don't worry your dog doesn't get sick by a simple sneeze just like humans.
Yes
The Inuits stored their kayaks upside down to keep dogs from eating the animal skins that the covers were made up of. The kayaks were lightweight, so they placed them on high racks that the dogs could not get to.
Dogs and cats
Animals Like Dogs, Cats, Bunnies and More Sneeze Through Their Nose.
because the way the dogs head is shaped
Same reason you do, irritation in the nose.
The same things that make a human sneeze....ie black pepper.
The earth can not be upside down.