Hello;

1st question:

I have

My opponent has:

QUESTION: Can I chose Jack and then return him to my hand?

2nd question:

Opponent has won 2 stories, he now wins 3rd:

Can he execute this story ability to make the game longer?

Question1: No, Jack Brady will have left -before- the ability resolves, so you have to pick another character to sacrifice if you return him to hand.

Question 2: No, the game is won as soon as someone wins a 3rd story, so you don't get to active it any more.

For question 1, no because Secret Murders is not a triggered abilities .

"Triggered Ability
A triggered ability is any triggered effect caused by a card already in play."

and "A triggered effect is any effect with preceded by the following text in bold: Action, Disrupt, Response, or Forced Response."

Secret murder have not words Action, Disrupt, Response, or Forced Response in bold in its text.