Wow. So are we all undersanding you right?!?
I think if it were my house (which it isn't, b/c I would never let it
get that far).., I'd do one of two things:
Pour 8'' of fill down (whatever is appropriate: 6" gravel + 2" sand, or
something, compacted real well), then put down a new 4" concrete floor
on top, bringing your floor back up to 6" above the bottom of the
Alternatively, I'd do your stem wall idea, but make the wall 16" thick
and 16" high, and pin it very securely to both the existing wall and
the new floor. This is a distant second place, though, because it is
unlikely the new floor perimeter was meant to take any significant
load. And, it would be a real pain to get them pinned together real
well -- I suppose you'd have a lot of drilling to do -- hundreds of
holes, at least.