Very nice story. I love the imagery and the way you made the story work. There are a few nits to pick, which I list below.

That transformation was nearly complete when the melt-down began and her ranting interrupted his voyeuristic pleasure.
This should be one word, not hyphenated.

"I'm done venting now, and I'm ready to go" she said as she finished zipping the shiny black boots that made feral creature stand several inches taller than perky housewife.
Should be a comma here.

It was a lyric from a song that had become a joke between them.
This sentence is phrased awkwardly. It makes the seque intop the next sentence a bit off pace. Try something like: A song lyric that had become a private joke between them, his recital...

He set cruise-control when they got on the highway, carefully shifted his hand to the back of her neck so he didn’t muss up the wig, and guided her head down to his lap.
Couple ot things here. Cruise control is 2 words. And this sentence is a bit busy for my taste. Not sure exactly how to fix it other than splitting it into 2 sentences.

And indeed she did. In seconds she had removed the cock from his trousers and was feasting on it. A few more seconds, and she was struggling to impale her own skull on the engorged head and shaft. Each of them exercised a different kind of discipline; she tried her best not to gag and he focused on not running head-on into a tree at 60 miles per hour. For a while the only sounds inside the car were Robert’s primal growls, vibrating down through Karen’s body and the gasping and slurping of the most perfect cock-sucking mouth ever created.
I would drop the initial and, leaving it off emphasizes the thought. The words in red need to have a comma after them, and I would add one after gag, just to help clarify. Also, delete the most. Adding a modifier is needed only in making comparisons, and modifying perfect is impossible, something is either perfect or it is not.

When Karen sat up and looked at him, the most wicked of smiles appeared on his face
Most wicked? Of what? This is why you only use these words in making comparissons. A hard habit to break, but a necessary one for a good writer.

Waiting there, dressed just like a Catholic schoolgirl was ‘Trouble’, also known as Amanda
You should use as here. Makes the sentence flow more natuarally, and is the best way to say this.

Finally the two women pulled away slightly from one another and turned to Robert, who was staring in wondrous delight.
Another missing comma.

As I said, this is a good story, and I like the way you tied it all together. the niots are nothing serious, and I hope to see more from you.