Thanks for the suggestions. It is going to be fun. Wish me luck.

I have never seen or even heard of such a crazy way to do a subfloor.
But still I wonder if you aren't making this too hard. Lets assume you
want to remove the concrete. You seem to indicate that it is sunk about
2 1/2 inches down into the joist bays, and then comes up about 1/2 inch
over the top of the joists. It must be resting on something since you
can't see the concrete from the crawl space. I'm going to guess that
there is some wood slats of some sort below the concrete, since that
1/2 inch lip would not hold the concrete so great.

Can you go into the crawl space and just cut out the rotten and
remaining slats. Being careful of course to not let the concrete fall
on you while doing this. After, you have just a 1/2 of concrete on top
of each joist. Bust that up, or cut it, and the whole bay would fall
down into the crawlspace.

But I am probably missing something given the convoluted history of
this thread. Good luck anyway. And get some pics will ya already.