I finally got the leak contained by sealing off both ends of the pipes that used to go into the slab. Here’s the manifold near the front door where the water was being run from the slab back down in the slab to the kitchen.
I had to do the second smaller cutout to get more room for the soldering.
You can see the cut and capped off pipe in the second picture below. I was lucky to find the supply line was a 3/4 inch pipe, and then a 1/2 inch teed off and went back down. Since there was a 1/2 inch in the kitchen, then that was the one. I would have had to guess otherwise.
The next set is where the pipe comes up for the kitchen and upstairs bathroom supply. 
The pipes on the left are cold water, and the hot water is on the right. There is a previous cap from an earlier slab leak a few years ago, so now both pipes are capped. My job is the left solder work, which I think is a better job!
The next sequence of pictures is where I dug into the backside of the upstairs bathroom and teed in the hot water supply line.
It was kind of a tricky job getting the copper tee just right. Basically you have to cut enough to be able to slide the thing in, but then slide if half a length up the other way and solder it. I chose to connect this pipe to the upstairs bathroom lines with PEX tubing. It’s a whole lot simpler than sweating copper pipes together. You can see my wonderful amateur plumber work up close.
So that’s about it. I still want to put up a picture of the other side of the PEX tubing. I just hacked into the existing hot line and even installed a ball valve!