In your pics you can see the bead rolled section that runs right through the rusted out area. Anything a few inches below the bead roll, which is there for structural ridgidity can be accessed from inside the car under the dash. Any damage above this would have to be cut out and repaired from only the engine compartment side unless you drill out the spotwelds and remove the cowl top sections. This illustration may help, but it is for a RHD. Just visualize the assembly reversed with the chimney on the other side for a LHD. I would cut out all the rust and replace the metal. Definitely more difficult to repair on a car that the dash is still in. You are looking at the tip of the iceberg. Rust settles in the seam where the cowl top sections join and are spotwelded to the cowl and quietly, gradually gets worse, absorbing & trapping moisture. After repairing the metal, sealing the seams, and ensuring the water drains, is very important.