Learn Italian: Il sovrano ha chiesto che nessun suddito gli disobbedisse. - The sovereign requested that no subject disobey him.

Even though the English translation says "disobey", the Italian main verb (ha chiesto) is in the passato prossimo, which anchors the sentence in the past. When reporting a command or request that happened in the past, the sequence of tenses dictates that you must use the imperfect subjunctive in the che clause.

Since this is a reported command, could we use "di" + infinitive instead of "che" + subjunctive?

Not with this exact wording, because the subjects of the two clauses are different (the sovereign asks, the subjects disobey). You can usually only use di + infinitive if the person receiving the command is explicitly the indirect object of the main verb. To use the infinitive here, you would have to completely rephrase the sentence, for example: Il sovrano ha chiesto ai sudditi di non disobbedirgli.

Why is the pronoun "gli" used here rather than "lo"?

In Italian, the verb disobbedire (to disobey) requires an indirect object. You literally "disobey to someone" (disobbedire a qualcuno). Because of this, you must use the indirect object pronoun gli (to him) instead of the direct object pronoun lo.

Why is it written as "nessun suddito" rather than "nessuno suddito"?