10" bolts with a hand drill in lead?
A good 1/2' electric with a low speed (500 rpm) and a lot of torque would be my choice. $500 or $600 where I am. Maybe you can rent it.
And if the plywood is wet all vbets are off - it should be removed and replaced with epoxy and roving to the same thickness.
As far as "especially when the builder says it will work" - remember the builder was the one who did it wrong to start with.