Ok, I've found the cause of the problem.
You must have:
var ShowingAllQuestions = false;
and I had:
var ShowingAllQuestions = true;
certainly because I had tested too much things.
However, I prefer to make changes in jquiz7.js_ rather than in hp7.cs_.
So the solution I use is:
// Addition: to display all questions at the start
ShowHideQuestions();
// hide the ShowAllQuestions button
document.getElementById('ShowMethodButton').style.display = 'none';
// End of addition
at the end of of the StartUp() function in jquiz7.js_
Cheers,
Agnès S.