I think I can help break down the first sentence:
First of all, let’s look for the general frame:
オープンまでの二週間、… ひたすら練習が続いた。
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:
二人組になったり、
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)
社員を相手にしながら、
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)
架か空くうの客に向かって、
facing an imaginary client,
So my understanding is that she is doing all of this.
Please correct me if I’m wrong! 