She really dropped the ball on that negotiation. They can hire double staff and then immediately lay off or transfer them, to fulfil the letter of the deal (and where is the signed written copies?!). And no guarantees to their quality, where iirc the reason she was only one doing overtime was because she was only person who cared about the quality of the work to the same standards?
And even with more staff she can end up swamped with work, both in terms of having to train them, as well as still having enough work to exceed it. Would have been better to receive a guarantee on her being exempt from needing to do any overtime (which technically she already is, but having it in writing so noone complains, since they are a black company).
The worst is probably however the fact that more staff means she's less valued - so she will likely receive paycuts or even get fired because they need to downsize and she requires the wage of a senior/manager position. Or simply because they want to ruin her working prospects as an attempt to force her into being an adventurer. So would need some guarantee of not being fired too.
And that is before even considering the magnitude of the reward: they clearly wanted her enough that she could probably have solved all her current problems in one fell swoop - damages house, overtime, guarantee of anonymity, maybe even rule-exception allowing her to officially/legally do adventurer work despite being a receptionist.