So reading around and looking in the KB it seems like Welded Corners are one of those features that in theory works but in practice rarely does so this may just be another one of those cases where it's not going to happen. But just in case a Sheet Metal guru is out there: any luck getting a Welded Corner to work in the four corners of the attached part (SW17)?
Hopefully the geometry / tree is fairly self explanatory (I've taken out a bunch of features for the purposes of sharing the file, all just hole wizards and cut extrudes on some of the faces). The Move Face1 feature is in there to get the face gap consistent at 1mm all the way round (variable gaps was one of the things my VAR suggested as being problematic).
I can get a Welded Corner to run along the long edge but not around the corner;
If there's a better way of modelling this up and/or manufacturability improvements I'm all ears, I've never really done much with Sheet Metal before.