1) As the "main page" can be different for every user I assume you're talking about:
In fact, the full URL is actually http://www.question2answer.org/qa/questions?sort=created
According to this line, the recent questions page are sorted in descending order by asked question creation date (or unix timestamp, actually). Note the created keyword is the value the $sort variable has when running this code on the given URL.
2) No, it is hardcoded, as you can see.
3) Yes, it is in the admin/lists section. It is the Length of Questions page setting. However, note this will affect the other subnavigation tabs (Hot!, Most votes, Most answers and Most views).