My name's Kel.
You are correct.
Do not place concrete directly on top of the French drain gravel.
Make sure it's tamped.
Then put some 6 mil poly over it then pour your concrete.
Your idea of using the cabinets to disguise the drop's a good one.
Here's my concern. Are you planning to have them step over the footing?
The footing is quite irregular in terms of height and projection.
Fitting the cabinet around will be quite tedious.
I'm guessing the result will look weak.
Why not just frame the walls on the new floor
almost touching the footing?
You'll lose some area -- in a single family detached -- about 40 square feet.
Enough for a closet
but you're space will be much more appealing
and it'll be easier to build than fitting cabinets.
If you want to have a smaller foot print -- that is -- not lose so much space -- you could frame the perimeter walls in light steel. Could use 1 1/4" or 2 1/4 studs. . .
What do you think?