Not sure if it's safe to talk here, but I'm thinking Draco is dressing up (polyjuicing) lilke his son's hero, and he's the one doing the dirty.

But then what would be the point of saying Harry doesn't know it. Of course he doesn't he's not there.

Then I thought, Scorpius is polyjuicing himself as Draco, and Harry is doing the nasty thinking it's draco, but then where does the insest come in.

Then the wand and the unforgivable, makes me think someone ie Draco, so they would be alone,just he and Scorpus, but still confused on the whole Harr doesn't know it yet sentence.

Please enlighten me!