Any additive at this point would be prolonging the inevitable at best.

The proper direction at this point would be to get it repaired properly. There is no such thing as 'mechanic in a can'.

True.
I have used the Lucas product personally and I can attest to the fact that it stopped the slipping in my 1977 Grand Prix TH400. It worked well enough that I drove it for a summer until I could swap transmissions. It won't keep your wife from leaving you or help you stop drinking, but it will help your transmission stop slipping.
I didn't believe some of the stories I heard about it, until I used it. My favorite one was about a guy that put it in a pickup that would no longer move, it slipped so bad. He had eased it to the side of a dirt road so that it was partially in the grass. He started the engine, poured in the Lucas stop slip, and got in the truck to try it. He had the wheels pointed out in the road a little bit, optimistically hoping for the best, and expecting the worst. He gave it a foot full of throttle like he expected it to slip, and it shot across the road and hit a tree.
Don't knock it 'til you try it, this stuff really works.

My wife even thought that was funny!

Which part, The part about the guy hitting the tree?
Or about how it wouldn't keep your wife from leaving you?

BOTH!!