Wood – Why is this plywood missing the grades

Don't worry about making new tabs or grooves. Just cut the offending tabs off. For the planks on the floor, use a sharp chisel. For the new boards, cut them on a chop saw. You want the two mating faces to be as perfectly square as possible, otherwise you will end up with a gap or one board riding up over the top of the other.

In places where you still have underlayment, a belt sander with a rough grit will take care of that in seconds. Make sure you have a shop vac hooked up to the sander!

Also, try not to have two planks next to each other where you have cut off the side and the end tab. This could lead to buckling. Provided you plan ahead on your installation pattern, this should be achievable.

Laminating a second layer of the same product should stiffen it substantially. Once the first layer is positioned the way you want, spread several beads of construction adhesive across the face of it and conform a second layer over the first. Tack in place with brads using a nailgun to hold it in place until the adhesive sets.

Note: These flexible plywoods come in a variety of thicknesses. It is likely that the thicker ones will be somewhat stiffer than the thin ones. I realize that even the thick ones may not be rigid enough for your application.