Page 19:
As mentioned before 先で describes a place and this place is “ahead”.
Page 20:
There was quite a bit discussion about this in the previous bookclub. I recommend reading starting from this post. From reading the discussion my interpretation of the sentence is: In the two weeks before the opening, we were grouped in to pairs and such, (and) while accociating with the (other) employee, we faced imaginary customers, (this way) we devotedly continued our training.
Few things that were brought up in the other thread: 相手にする is a set phrase meaning to accociate with or to deal with or to keep company. There were others which also pointed out that AをBにする is a grammar construction which means to make A to B. So here this would mean to make the (other) employee a partner. Personally, I find reading this phrase as 相手にする wiht an を object more fitting. But I wanted to give you options. 
Page 21-1:
As mentioned before, と is used as a quotation here. Think about it this way: The new hires were probably told “You need to get used to handling money, so we put some in the chash register”. And now the protagonist tells us about what was said. That is why と is used.
Page 21-2:
I interpret this as: because everything is lined up so neatly and there are still no costumers, all looks like fake products. So basically, as if this were a fake store with all fake products, becuase it is (still) too perfect, too neat.