Section A-A is what you would see if you sliced the part along the dotted line and see it from the direction indicated by the arrows.
If you cut your part along A-A you would see a rectangle, not a trapezoid.
edit: did my own model and added a picture for reference :)
The part in contact with the old seal is about as wide as the seal, the part in contact with the seal groove is narrower, as it should be.
The tool as shaped by the OP would get wedged between grime and fragments of the old seal or might cut the old seal because of the too narrow point that only contacts the old seal in the middle or on either edge.
i.e. in 3rd angle projection, the section view would be on top and not at the bottom of the parent view.
Hopes this helps to clean up some of the confusion.
The alignment of the section view is off as well. It should be aligned with the parent view:
I wanted to publish it earlier but had to wait a day before I could publish it
I play with OpenSCAD for fun, but because I use Inventor and Rhino at work, and FreeCAD at home, I don't use OpenSCAD to actually design stuff.
If I want to do generative, creative and coded design I use Racket with the Rosetta package  with Rhino or AutoCAD as my backend. I am trying to port Rosetta to FreeCAD , so it is all free. Racket with Rosetta is a lot more free or open to creating generative designs than OpenSCAD. I really get a kick out of Grasshopper  with Rhino too.
This sounds really interesting. I've been wanting to work with Rosetta but with a backend other than AutoCAD, hopefully something that could run in Debian. I'd be interested to follow any progress you make on this front if there's a code repo / link you could point to, or maybe message me (contact info is in my profile).
Doing it all by trig seems like a lot of work too, a good parametric CAD system would have higher level primitives for curves, chamfer, fillet etc.
Also, I am gradually acquiring a sense of space, physical effort, and mechanical operation of things in the real world after a long time in software and feeling like I “couldn’t do” these sorts of things. It’s sort of an eye-opener and a confidence booster to even get through something like this the first time, however sloppily.