What you have is a clash of expectations. Your group likes to plan and strategize, she clearly doesn't (or rather, planning and strategizing is less important to her than playing this impulsive character). It's really important to articulate the expectations of play and you should do talk to her, and the group, directly, rather than create an NPC to try to block her character.
You'll notice if you look at other media or stories, there's a big difference between impulsive characters who TALK about jumping in, but don't actually do so, vs. those who DO and cause a lot of chaos.
If your group expectations are that everyone is mostly working together and only conflicting/at odds for show ("The elf and the dwarf are arguing again." "Oh, let them be, they're actually good friends.") and she is operating on the idea that each character might have very different agendas or less concern for the group, you're going to have clashes.
Being clear about what kind of game this is, helps her decide if this is the game she wants to play or not.
I have a lot of experience with players that act that way. I've had a player like that, at one time or another, so often that it sounds like you're describing my old games.
The one thing I will mention is that this is obviously something out of character related. She is frustrated, annoyed, angry, depressed, or some other emotion about something. And she's taking it out on the game. Since you only have four people and one of them has to be a DM, it's a hard situation to deal with.
This gets a bit into interpersonal skills and might not really be the best place to talk about this, but since you seem to be having a problem, here's what I can offer.
She doesn't want to play the game
It's very obvious from just the first two instances you posted that the player genuinely doesn't want to play the game. Playing a game is about getting into character and rolling with the punches. No plan will be perfect, your character won't know everything, and you're going to get into situations that you are not prepared for. Dealing with all of that is what makes the game fun. It's clear that she just wants to auto-pilot and coast through every scene. And you throwing wrenches into the plans is causing her to get aggravated. This is because she doesn't really want to 'play'.
She doesn't know what it means to roleplay
Maybe she's new to role playing completely, which is an option. If that's the case, she's having a large disconnect with what she 'thinks' she knows and what she actually knows. There is something to be said for a character actually having knowledge of a situation. But then, there's the world-building aspect. There are certain things that the DM will decide that the player has no control over. In fact, this should usually happen. And a player can discuss with a DM about the things they do and don't know or how they should and shouldn't act. But it should never be a case of 'I would never do X' when it's clearly against her character.
She doesn't want to play your particular quest line
I don't know your party dynamic, but it seems like she's poking holes in your game without reason. There is ample place for a party to get involved and have fun doing something in the scenario you've set you. However, she's actively making a character-breaking decision to not go along with the party. The group around the table should NEVER have to discuss for hours about going one place or another. This is a side affect of trying to pull a stubborn mule. The mule doesn't want to go and is going to make everyone else miserable as they try to drag it somewhere it doesn't want to go. And that's what she's being, a stubborn mule. It seems she doesn't want to play your particular side quest, for whatever reason. Maybe she liked the other DM better? Maybe she has something against you? I really can't say, as I don't know your friends.
The Solution
Since you have such a small group and you guys seem to be in the middle of a campaign that someone else is technically running, your options are limited. I would tell you to just let her know that her actions are pretty destructive to everyone's fun and inform her that she's not welcome if she's going to be a petulant child (use better words then that). But, you'd be down a player in the middle of a campaign, and that could hurt your friendship and game.
My suggestion would be to sit down with her, one on one, and talk to her about what's bugging her about the game. She might go over the little details and reiterate what she's said in game, but try to push past that and get to the root of the problem. See what she's really upset about. Maybe home troubles? Maybe someone at the game is frustrating her? Try to figure out possible ways you can tailor the game more to what she's feeling. Maybe she's sick of playing the previous type of character or it's not what she was hoping for. That happens. Try to work with her to either play something different or focus the game more on what she could like doing (keeping in mind the other players interests as well).
One thing I wouldn't suggest is an intervention. Those end badly, simply because the player will feel everyone is ganging up on her and alienating her from the group. Try to keep it one on one with her and let her know that you're just trying to help her have a good time.
Best Answer
Off Table
I will always advocate discussing these kind of issues with your players outside of the game. Find the reason why your problem player has built his character this way and what his end goal is. That way the two of you could work together towards it.
If it is a case where it is just going to continually disrupt the game, give the player an option of creating a new character and letting the "greedy" one leave the party in a way that makes sense. Maybe even coming back as a villain later proving the evils of greed.
On Table
How are they carting around all this gold? Coins are bulky, make a lovely clinking noise inside a purse, and also a loud rattling noise when kept in backpacks.
Have the party constantly plagued by beggars, shop keeps, religious orders, etc. This won't dissuade them from seeking more of a reward, but smarter ways to carry it. This might mean, from your example above, they may be more willing to accept the heirloom without asking for a coin bonus.
Their greed could then (if it continues) start to have an in-game effect. Divine characters could start receiving attention from other gods. For example a cleric of Bahamut has just used a divine spell:
Primal characters could start seeing visions of extinction or other such horrific consequences of imbalancing nature due to greed.
Martial, Psionic, and Arcane would be more difficult to take this approach but maybe something specific to their backstory could inspire something. Finally start to change alignments, and have the effects of that trickle in. If that is the approach you go with, prehaps be ready to shift to an evil campaign.