A scientist is studying bacteria whose cell population doubles at

Dear Mkappa,
I'm happy to respond.

First of all, I believe you are interpreting the first quote incorrectly. The stem indeed says
Four hours from now, immediately after the population doubles, the scientist will destroy the entire sample.
You are interpreting this as the first and only doubling period, which is not justified by the context.
Let's say, right now, it's 2:00 pm. Let's say that the doubling period is every 30 minutes. That means it will double at 2:30 pm, then at 3:00 pm, etc. It will double at 5:30 pm, and the scientist will destroy the sample at six, but this statement explicitly lets us know that the destruction at 6:00 pm will take place after the doubling at 6:00 pm. In other words, the amount destroyed at 6:00 pm is the amount after the 6:00 doubling, not the amount after the 5:30 doubling. That was a potential ambiguity in the statement, and the MGMAT folks wisely clarified this subtle point with this statement.

You are also misinterpreting the second statement you quote:
The population will double to 40,000 cells with one hour remaining until the scientist destroys the sample.
Say, the time of destruction is 6:00 pm. Say, there's a doubling event at 5:00 pm, and after this doubling event, there 40,000 cells. Does it double again between 5:00 pm and 6:00 pm? Maybe, we don't know. Does it double again at 6:00 pm? Maybe, we don't know. Does it double more than once between 5:00 pm and 6:00 pm? Maybe, we don't know. We can't infer the doubling period from this statement.

My friend, I gather that you are a non-native English speaker, and it's perfectly understandable that the sophisticated and subtle wording of a word problem confuses you. It's important to be patient with yourself as you learn. I will suggest this blog:
https://magoosh.com/gmat/2014/how-to-imp ... bal-score/

I want you to consider something very important. GMAT Club is a public forum, so anyone could see your words here. Other readers of GMAT Club now might one day be your boss, your co-worker, your business partner, your customer, your supplier, your consultant, etc. You never know where you meet some of these people again in the business world. It's very important to focus on making a very good first impression: psychologist tell us of the tremendous power of first impressions, and you never get another chance once you made one. In your post, you were very critical of MGMAT:
So now I am confused about the usefulness of MGMAT. Plus it has some nonsense questions where non-convex polygons come into play.
Professionally, the MGMAT folks are competitors of my company, Magoosh, but I will unreservedly say that MGMAT is one of the best GMAT prep companies in existence. Those people at MGAMT are truly brilliant individuals and wonderfully gifted teachers. They command universal respect in the GMAT preparation space. You misunderstood some of the statements in this question, but rather than assume that perhaps your reading was not perfectly correct, you immediately went to criticism and blame. To be honest, my friend, this does not make you look good: it does not look good at all when you misinterpret a shortcoming on your own part as a reason to blame somebody who is widely held in respect. I am telling you this, because I want you to be successful. I want you to make a good impression on others professionally and have a wonderfully prosperous career. Especially when you are in the position of a student, humility and cautious questioning go a lot further than automatically criticizing and blaming. It's somewhat anti-intuitive, but genuine humility can allow you to go very far in the business world: in many ways, for example, Warren Buffet is a genuinely humble man, and he's one of the richest men in the world.

My friend, I hope you understand that I am pointing out this difficult things precisely because I want to see you thrive and be successful in all aspects of your business school application and your business career.

Sincerely,
Mike