I think I can help break down the first sentence:
First of all, let’s look for the general frame:
In the two weeks before the opening, … devotedly continued the training.
Who is doing this? Our protagonist.
Now how is she doing it? That’s what is in between:
they formed groups of two people (this is not a literal translation, になる means “to become”, but I’m having a hard time finding a good English tranlsation)
here, 相手にする means e.g. “to associate with”, so this gives us:
while associating with an employee, (my understanding is that they form groups of two new part-time hires which are then accompanied by an employee)
facing an imaginary client,
So my understanding is that she is doing all of this.
Please correct me if I’m wrong!