Is it permissible to supplicate against oppressors for them not to be guided, by analogy with the supplication of some prophets against their peoples, or is it obligatory to supplicate for their guidance since they are Muslims?

It is incorrect to infer from the supplications of Noah and Moses (peace be upon them) against their peoples that there is no desire for guidance for disobedient Muslims. This is because their peoples were disbelievers, and their supplications came after they knew that no good could be expected from them. As for the Muslim, he is not obligated to supplicate for the guidance of the oppressors; rather, he is permitted to supplicate against them without transgression.