BridgeMan

Segmental walls would be better (and easier) than building a wall on a concrete footing below frost line. And a lot faster. Save your concrete form-building (and placement and finishing) efforts for the patio. And don't forget to slope the surface (and forms) at least 3/16" per foot, away from the house (meaning a 10'-wide patio will have a total drop of close to 2").

joecaption

For future referance never use pressure treated plywood for sheathing.
It's now treated with copper and will rust out anything but ACQ approved fasterners in about a years. Reguler roofing nails are not ACQ approved.
With yours I think I would have removed the siding and cut that sheathing back up higher on the wall. At least I would have removed 2, rows of siding Installed 3/4 X 6 vinyl lumber, Z molding. a strip of J molding then reinstalled one row of siding.
This would keep the bottom of the sheathing from being exposted to wicking but still covered up some of the concrete wall.