Translation:A lot of people had stopped to talk to her.
The first option: stopped to talk to her. "Stopped talking to her = parado de falar com ela. He stopped TO smoke = ele parou PARA fumar / He stopped smoking = ele parou DE fumar.
There's no difference in meaning in English. "...had stopped talking to her." is preferable.
I guess you are right but it seems to me that "a lot of people" should be plural. But Duo sees it your way.
Why DL does not accept: "... stopped in order to talk to her" ? They themselves suggest "in order to" in their translation.
I believe "a gente" can be used to say "us" also.
How would one say "many of us had stopped to talk to her"?
"Muita a gente...?"