Fun story! I really love Fred as a ghost. REALLY REALLY brilliant! He could be the Peeves of WWW! LOL
Personally I don't think Harry had to use his 'wand' at all. I think Kreacher kept changing Severus back to his animagus form (note how he suddenly changed back when Kreacher needed to replace Harry's teddy?) Yep, Kreacher, the voyeur, was just watching while they had sex and at the right moment, he created a blue light around them with elven magic!