I see two problems with the situation where you're using social conflict to get the clue from the worried mom type.
1. I don't think it necessarily needs a full scale social conflict. You wouldn't need to run her out of Composure and drill through Consequences. Assuming the PCs were at all sympathetic, she's probably Concede pretty quickly. Most likely after taking a single Consequence. There's no need to draw it out more than that.
2. You're making a mistake that I find a lot of people new to investigation style games make: You've bottlenecked the information at one point, and the PCs need to do this one thing in order for the game to move forward. Failure shouldn't mean that everything stops; it can easily mean that they get the information they need, but that something goes wrong in the process.
There are a couple of ways to make social conflicts more effective.
The simplest is to build a sort of support structure. One PC acts as the voice, while the other PCs Meneuver to create temporary Aspects and pass off the free invocation to the voice. This boosts the effective Skill level of the acting PC and involves the rest of the group as well.
Example Maneuvers:
* Making tea for everyone, putting the mother at her ease. Example Aspect, "Camomile puts me at my ease."
* Saying comforting things that support the points of the acting character. Example Aspect, "We really just want to help."
With two characters supporting the speaking character, even with a 50/50 chance of each Maneuver working, the acting character gets an average bonus of +2. If you can manage Maneuvers that wouldn't logically be resisted by the NPC, that jumps to an average of +4, which is huge.
The group social conflict thing can be sticky. I'd judge how things should work on a case by case basis.
If there's one strong leader and the rest are mooks, the leader's Concession should be enough to end the conflict. That one leader is the driving force behind that side's conflict.
If it's a group of equals, you might have to push through several people. Much of the time, once a fair portion of the team has fallen, those remaining will likely want to Concede in a way that lets them leave, perhaps with those taken out. Sometimes you'll have to drill through everyone, though.
A very rough guideline might be something like this: If at least half the people who matter are taken out, the rest of the group will try to jump ship unless the other team is at least as bad.