[Shostak]

That's effectively a 10 point bonus/penalty, which may be too much. Rather than have demons be so predictable, why not give them a minimum IQ (10? 12?) and randomly adjust it upward--say by 1d or 1d+2?

You'd still have the issue of the demon blasting the wizard across the barrier of the pentagram. Here's one way to deal with it that approximates what happens RAW: the lost contest voids the summoning and the demon returns to its own plane, to which it immediately summons the wizard, who, now bereft of the pentagram' protection, is instantly executed. If the wizard wins the second contest the demon throws the cadaver back into our plane, where it could theoretically be resurrected.