What provides the correct compound subject or object to complete the sentence and correct pronoun case We were stunned to come home and find that Mary had made a beautiful chocolate cake for?

There is no compound subject.

The subjects of the compound sentence are:

• We, subject of the verb 'were stunned'
• Mary, subject of the verb 'had made'

There is no compound objects. The direct object of the second part of the compound sentence is:

• cake, object of the verb 'made'

The only thing missing from the sentence is the object of the preposition 'for'. The missing pronoun is the first person, plural, objective pronoun that will take the place of the antecedent 'we'.

• We were stunned to come home and find that Mary had made a beautiful chocolate cake for us.