Imparfait vs passé simple

Question

- Can I use "se composait" instead of "regroupait"?
- why use "en eut assez d'attendre (passé simple) while this place is supposed to describe the speaker's feeling, therefore can I use avait (imparfait) in this context.

Accepted Answer

I think ‘se composer’ fits, (and checked with a native French speaker who spontaneously suggested this as one of a few possible verbs, which interestingly did not include ‘regrouper’). As always, there is usually more than one way to express ideas, and translate phrases and not all can be covered in a fixed exercise. 
To your 2nd question, the lesson instructions note to use passé simple when appropriate, which in the text will mean to use passé simple instead of choosing passé composé. 
The English phrase is ‘One day, she got fed up with waiting ..’ - this is a specific “event” that happened at a specific time and would normally be described in passé composé, and therefore passé simple is what is expected here.
It is not a general description of emotions or feelings, with vague or fuzzy onset and offset, nor basic background information, and using imparfait doesn’t fit with ‘one day … got fed up….’,  and would not convey the same meaning.

Answer

Bonjour Thanh Tân,
Maarten's answer is really good!
Just to supplement Maarten's answer, the difference between L'imparfait and Le Passé Simple is that L'Imparfait implies a certain duration (ongoing in the past) while Le Passé Simple implies a very brief action (one-time completed event ; just like Le Passé Composé).
I hope this is helpful.
Bonne journée !