Always more than one way to approach. Of course I only have pics to go by. I don't have a stud welder. I have had situations with pretty deep dents, as that seems to be, in a place with no rear access. So I probably would cut out the dent as a rectangular piece going up somewhat. Then I would form the piece back to shape. I Then I would find a suitable tool to stick up in the hole to force the seam back together. Then weld the fixed up patch back on.
I may take a good look at the area where the window frame is. Inside. I may cutout a piece to be able to fit a tool on there to force/pry the dent out and the seam closed. Thinking that the window frame will cover much of the patch. Not sure about that idea. Window needs to come out anyway, so take it out for that repair. I do like to try and think of as many ideas as I can, crazy or not, to get outside the box thinking going. In my past career, it's called brainstorming. To be sure I had thought of all possibilities and not get fixed on something just because I had a stud welder. For example.
You can obviously cut a hole and go straight in from the back to pound the dent outward and the seam upward. The piece you cut out will go back on and be the proper shape and fit. You need to use a thin cut tool so you don't have gaposis....and be able to weld thin metal. (I have a suggestion for that also if you decide to weld)
I have done both the ideas mentioned.
Nothing is easy. It's all a lot of work. For me it gives satisfaction (once the grinding and filler is done).
The stud weld way could be the best. I just don't know if the seam will close up....and can't tell the extent of the situation.
It looks like you do really good work. Kudos.
