To answer your first question, although he hasn't played 50 over cricket internationally, he has been involved at domestic level. He took part in the Royal London Cup this year for Hampshire. As long as he is signed up for a team, which in this case is UBL, he has every right to represent when he wants. Also, I am fairly confident Afridi would have been approached by someone at UBL to take part and not just demanded a spot in the team.
Regarding your second question, I am not sure if you follow English football, but during the early rounds of the FA Cup and league cup, teams rest players and give youngsters opportunities. It is not until the latter stages where the main players will start to be selected.