First off, I want to say that the answer to every one of your questions is yes and no, and that it all depends on the individual bird. Let me explain.
Are these birds extremely loud?
My female, even when she is making noise, is very quiet. If she was in a different room than you, you wouldn't even know she was there. My male, on the other hand, if you were on the next block over, you would know he was there.
Can these birds bond with more than one person?
In my experience, it depends on what you mean by bond. With my female, my wife can take her out and pet her, and just generally interact with her with no issues. However, I am her person. If she is with me, she will have nothing to do with anyone else. So if you're just trying to see if more than one person could interact with them, sure, not problem. However, most of that depends on you. The better you socialize an ekkie, the more social they will be. That is true of any bird.
Are they extremely messy?
Again, depends on the bird. My male is pretty easy to clean up after. The couple bars on his cage that are next to his bowl have to be wiped off daily, but that is about it. I do feed my birds almost exclusively fresh foods, which does make them messier, in my opinion. My female, however, is by no means a neat bird. Half of her cage has to be wiped down daily, plus the floor all around her cage, under her cage, and in other random spots that I'm not even sure how it gets there, but it does.
Are these birds difficult?
I guess this depends on what you mean by difficult, and what you're starting with. Both of mine were rescues, so they both required more work than they would have, had I got them as handfed babies. My male is blind, so he requires extra care when it comes to handling and cage set up, but I wouldn't say he is difficult. My female can be very pushy and intimidating. I do not find this difficult, but I'm sure a lot of people would. They can be difficult, compared to some other species, in that their diet is so specialized. These are not a bird that you can just give a bowl of pellets to. Not that you should with any bird, but ekkies are extremely diet sensitive, and even if you do everything right, you can still end up with issues like toe tapping.
When they bite, do they bite hard?
Duh. Sorry, I couldn't help myself.

They are a bigger bird, with a big beak, and they know how to use them. A lot of how hard they bite depends on training, and why they are biting. Do they have the capability to bite hard and do a good amount of damage? Yes, very much so. Ekkies also are a bit like the pit bulls of the parrot world, in that they have a tendency to bite and latch on. As they use their beak like a pair of scissors and try to saw through your hand. I know, been there, done that. However, with any bird, you WILL get bit and some point, and you will probably get bit good. It is something you must accept before you decide to bring a bird into your life. Even the little guys can give an awfully painful bite. I think my least favorite bite (is there such a thing?) came from a cockatiel.
Can they develop bad habits?
Of course they can, just like any bird can. A lot of that comes with the individual bird, and their environment. The less stimulation a bird has, the more likely that they will develop bad habits. Often, bad habits develop because of boredom. Again, this is true with any bird species, not just ekkies.