Rust repair is a slippery slope - I did a ton of it in my Michigan days. I agree that the repair area is almost certainly considerably bigger than the obvious flaw. Rust is like an iceberg with the majority below the surface.

I'd clean the snot out of that, making sure that the edge of the remaining metal is full, original thickness. Then make your patch. Brazing will work just fine and takes less heat and so less warpage is likely. You'll need all of the edges to be REALLY clean whether you weld or braze it. The better you fit the patch the better the outcome though if you braze it you really need the patch to slightly lap the hole as it's difficult to get a decent edge bond with brazing. Hopefully you'll be able to pull the lapped patch up from the bottom of the hole - just attach a length of welding rod to the front face of the panel to use as a handle. You'll cut that off and grind the area when the patch is in place.

There IS one other option. IF you can get the area REALLY clean both top and bottom you can bond the patch with an autobody structural adhesive. I have the Lord Fusor system and there are others. This uses a sort of caulking gun arrangement with 2 tubes, 1 adhesive and the other the catalyst. It mixes in the nozzle, which you toss after each use. A lot of modern cars are essentially glued together so this makes a completely satisfactory repair. The best part is that zero heat is applied but you do need a lap for the adhesive to have a place to be. If you can get the patch below the existing metal (like the brazing process) you can fuse the patch in place and come back the next day with filler as needed to complete the repair and make the window channel even with the existing metal (you could even use JB weld or similar for your filler). If you don't want to invest in the adhesive system you ought to be able to find a body guy who would stop by your house in the evening after work and bond your pre-made patch to your prepared body metal - might cost $25 or $50, something like that.

Wish you lived in my neck of the woods - we'd be done and sharing a Diet Coke by now.

Dan