First off, will Jenny get bad ideas if the other dog isn't as well trained as her? Say, it doesn't come when called, will Jenny stop coming when called?
For the most part, the new dog will learn from Jenny. Although, when I got Sam and he would chew on things he's not supposed to or pee in the house, Molli started some of those bad habits she never did before.

What changes? Like what is the difference, walking one dog compared to walking two?
I think it's just as easy to walk two dogs, as long as they are both trained to walk politely on the leash. I taught Sammy to walk on a leash by himself before I started walking him and Molli together.

Will it not be as attatched to me as Jenny is?
Depends. If you spend a lot of time with the dog, then he/she may be just as attached to you. It also depends on the dog. Like mentioned before, some dogs get attached to just one person, some love everyone.

Now, whenever we watch a movie, Jenny will quietly lie on her bed. If we got another dog, would they both lie down, or would they be up playing with each other?
This also depends. If they got enough exercise for the day, they will probably sit quietly, or you can easily get them to settle down. If they've been cooped up all day in the house, they're more likely to play with each other. Also depends on their mood and temperment. Molli and Sam love playing with each other all the time in the house, but I can usually get them to calm down when I need them to be. Sometimes one or the other are in a grumpy mood so they won't play.

How do you keep the fastest eater from fighting for the others remaining food? Or would they not be able to both eat in the kitchen? (my mom isn't fond of the idea of them not eating in the kitchen, because it would ruin the carpet)
I would strongly recommend you keep them in seperate rooms or their kennels when they are eating. Many dogs are food aggressive and that could cause a fight. Molli is very food aggressive, so I can never leave them together in a room with food or treats.

Would they fight over the 'best' seat in front of the fire?
Most likely not. If they're close they may cuddle together. If they want their space, one dog would find a different spot.

How do you tell the difference between play, fighting for dominence, and 'real' fighting?
I dido what Kay said.

And a biggie, because I've heard a lot of different opinions on this one: Are two dogs not much more work (than one dog is) or a LOT more work (than one dog is) ?
I find that there is a lot more work in some areas like grooming, training, etc. I find that I really don't have to exercise my dogs nearly as much, because they wear each other out from play fighting and chasing each other. It's also nice because my mom works 12 hour shifts and I go to school, so they keep each other company while we're gone. If your dogs are a good match, it shoulden't be too much more work than having one.
My dogs are quite a bit of work because of Molli's severe food aggression, and they still have not figured out who is dominent over the other so they fight quite a bit.