Yes, you share responsibility for this campaign. However, this is not because you encouraged her to DM, it's because you are playing in the campaign. As such, yes, you should either leave, or instigate some changes in the group. In the end, you really shouldn't be playing unless you are enjoying it.
However, don't try to blame the problems on your DM - the whole group is responsible.
You have a number of options. Leaving the group is definitely one of them. It comes far ahead of just playing on without enjoying yourself. However, it's still pretty close the bottom of the list of options, I'd say.
Changing the current game
You say you have tried to "teach her best you can". However, you shouldn't make it about what she has to do and what she is doing wrong. In fact, it's better to leave out completely what one should do when running a campaign.
Instead, focus on your current campaign only and tell her what you are not enjoying about the campaign. Don't tell her what to do or what not to do, just tell her about the elements the group isn't enjoying and see if you can let her come up with a way to make it more enjoyable for all of you or see if you can come up with solutions together.
At the same time, change your own attitude about the game. The details really need not matter as your groups seems to make it seem. For this, let me handle the examples you gave.
[A] player kept asking why the population of a city we had just visited dropped by 20,000
Possible answers include:
- "You're right, I got the number wrong, it's actually 20.000 more than I just said."
- "I got the number wrong last week, it's always been 20.000 less than I said last week."
- "Yes, there are a lot less people than last session and it's not immediately clear how this came to be. Don't ask the DM, ask the people in the city." Followed by the people of the city not being aware anything is out of the ordinary and the true reason only being revealed if they decide to investigate it further.
Each of those answers should suffice and could be the end of the story, letting you continue enjoying yourselves instead of getting bogged down in discussions about details.
I questioned why something barely larger than a lake was an ocean
This case is similar. If it is about being called an ocean, the DM only needs to tell the players that is just what the people call it. They can ask a random NPC. The NPC might tell a story about how it started in jest or how the place used to be larger, or the NPC might not know why it is called an ocean. Then if he continues to ask about it, the DM could suggest that they could seek out a historian if they really want to know.
If, instead, it is about the magical properties of an ocean, the whole thing can be more or less the same. It just acts like an ocean magically. This time, though, they would have to find a magical expert to find out why this place acts like an ocean.
Stopping the current game
When this doesn't work, I think it's better to do things as a group than to just quit yourself. This way, you will be able to keep playing as a group. As such, I would suggest you talk to the group and if everybody feels the same as you, you can suggest to stop playing this campaign and play a campaign run by someone else instead. You could break it to your DM somewhat like this:
Hey DM. I talked to the guys and not all of us are really enjoying the game you're running. We're going to run another game and you can join if you want - we just wanted to play with another DM for a bit.
It is definitely true that this may still feel like betrayal to your DM, but if everybody really feels as bad about this as you, the alternative is the group slowly falling apart. And that's worse in my opinion, because it may cause more rifts in the group.
Leaving the game
If the others don't want to start a new campaign for some reason or another, another option is definitely to get out. And as others have mentioned, leaving does not need to be considered a bad thing. You weren't enjoying yourself and you are stopping that in order to enjoy yourself.
If others turned out not to be as bothered by the DM ethics as you were, it may improve the atmosphere during the games. Either way, it may save your friendship with the DM in question.
This is definitely a better step than just to continue playing while not enjoying yourself. That said, I would try the other options first.