2 is really not possible here.
The dative without a preposition is used to mark the instrument with which an action is accomplished, but only with the perfect tense (generally) is the dative used to mark the agent with a passive verb, i.e., a being endowed with the capacity to act of its own volition, as opposed to a lifeless thing. (In other contexts ὑπό + genitive is usually used to mark the agent of a passive verb.)*
The complement of σπένδω (if it has one) is typically a dative indirect object -- to pour libations to someone or something.
In any event, ἔξεστι is really not a passive action verb that could take an agent as a complement. The English translation "it is not allowed" is a little misleading in this respect because it's passive and it can take an agent as a complement in English: "it is not allowed by X." A translation that better reflects the Greek would be "it is not possible" or "it is not permissible."
*In the case of the perfect, "agent" is perhaps not the right word. The perfect generally denotes a present state resulting from a past action. Thus, "the party responsible for the current state of the subject" might be be a better description than "agent". But this is too technical and it's irrelevant to Mastronarde's sentence.