To the OP: Being commanded to remain standing is pretty rude. Why should that girl cooperate with someone who probably won't appreciate her compliance?

I always choose my seats for specific reasons. If I were that girl, I'd have politely told that man I wanted my assigned seat. And I wouldn't have allowed that mother to put her bag under her seat. I always "utilize the space under the seat in front of you", so I'd need that space for my own bag.

Now, if a flight attendant made the request nicely, or if the man had been polite, I'd be more willing to give up my seat. But I'd check to see what seat was being offered in return.