Here is how I would write those sentences more naturally. Of course, I am aware that some of the expressions I use here may be too difficult for your students. As I already wrote, what you have is fine for a test.
<1> 7. "I (realized / discovered / found out) that I had met him (before / previously)."
<1> 8. This is not the kind of thought we would typically express, so it is hard to express it in a natural way. Exactly what we would say depends on the specific context of the situation, and what information we are emphasizing. A couple of my attempts at saying this are:
"I first came to this museum before 2010." "I had already been to this museum by 2010."
<3> 1. Again, how we would express this depends on the context. If the decision has already been made, but the father has not yet gone to the country, we might say: "My father has decided to go to that country." or "My father has decided to live in that country." If all of the events are in the past (for example, the father has already been there and come back), you can remove "has" or change it to "had" in the previous sentences.
We don't seem to have a simple word in English which conveys the idea of living somewhere for an extended but non-permanent period of time. The word "sojourn" fits, but is not so commonly used. "Stay" is OK, but also has a different meaning ("not move from one's current position"), which can cause confusion.
I recommend that you keep the sentence that you already have, rather than change to the one about becoming a soccer player. It's fine for an English test.
I am sorry I cannot be more helpful...