Is it permissible to make the supplication (du'a) after the 'Asr prayer if I performed the two rak'ahs of Istikhara prayer immediately before it, or must the Istikhara prayer be repeated after Maghrib?

There is no need to delay the Du'a of Istikhara, as its words are simple, and reciting it before the prayer does no harm. If you omit the Du'a and perform the prayer first, that does not prevent you from reciting it after the prayer, because this is a slight separation that does no harm. Al-Shawkani said: "Delaying the Du'a of Istikhara from the prayer does no harm, as long as the separation is not long." And if you repeat the prayer and the Du'a, the matter is flexible.