My car has some rot under this area and I'm thinking it might be easier to weld the wing to the scuttle along the join. There are a couple of pin holes leading to what I think is a small cavity underneath.
I don't really want to chop up the wing in that area as it looks quite a complicated join of about 3 panels.

Poking around underneath found some cracks in the seam sealer with rusty residue in there. After removing the seam sealer there is a small hole here leading to the inner wheel arch

And another here under the scuttle panel by the trunk hinge....I think all three areas are where moisture has crept under the seam sealer and rotted it out from the inside.
Other than this area and a few small holes in the floor, I'm quite pleased with how solid the car is.

I personally have not done this area.. but the body shop I use has done this numerous times. They normally start with a good piece and cut out the rotted stuff. Yours is small enough that somebody/you could hang upside down and weld this up.

You are correct. This is a highly technical area with all the pieces coming together.



B

One way to patch up each of those rust spots would be to remove the fender completely, but probably not worth the effort for the small size of each area.
I don't like the idea of welding the fender and cowl (wing and scuttle) together across that opening.
Sometimes I hold a piece of graphite against the back side of a hole and mig weld until the hole is filled. The graphite helps control the heat build up and doesn't stick to the weld, leaving a fairly flat surface on the backside of the weld.