Bonjour Eric
Ok Eric, I think I found it, the way to solve my pending points. Do you have more of this top secrets in your app?
It's fine to use "Convert to lines", converting an arc to a set lines. And after to use "Convert to polygon" converting the set of arc lines in conjunction with my straight lines to that what I asked for as shown below.
The second picture shows a copy of the first arc, moved down and was vertically mirrored. All this was done before breaking to lines. But the result is something that needs some care from your side. I think so, maybe I'm wrong.
It's really not a problem for me, that the process to convert an arc to a sett of lines can't be reversed. I stored the original arcs at another layer and hide this layer. If I need to revise my drawing this will work.
Cordialement
Hubert