Can you tell me which of the following three questions is best formulated?

1)Who mentioned Kurtz to Marlow for the first time? OR
2) Who was the first to mention Kurtz to Marlow?
3) Who first mentioned Kurtz to Marlow?

I don't think "entertainer" is the right word to describe the part-time job done by students during their summer holidays.
1) In June I worked as an "entertainer" at a summer camp. I had to look after children aged between 5 and 13 years old. (In particular, I had them play football, table tennis and other ball games).
2) I took out a monthly subscription to the local swimming pool (can you say that?)
3) I enrolled in high school (or I entered high school?) in 1998.

I like #3 best.

I would use the term "sports counselor".
Use "membership" instead of subscription.

#3 Either of those are used and correct.