1. The idler pulley helps the belt wrap more around the alternator pulley and the PS pulley, so YES, it would be harder for it to "slip" off.
2. There is NO WAY IN H3LL that a belt is tight enough when it goes over the idler pulley and still be long enough to fit properly under the pulley. It commands a longer size.
Before I use a socket to loosen the tensioner, the right size belt lines up with the center of the STAR hole on the idler bolt. Then, when I loosen the tensioner, the belt slips under. There is no way that that belt will fit right both above and below the pulley.
RC