As an example, I created your model as 2 solids "pre-boolean" and sent it to a program that uses the parasolids modeling kernel. Powershape.. Importing the parts in it first starts me through a process of setting up "tolerances" that I want it to use on the part.. Something like a 6 step process.. Then I booleaned the 2 parts together and it started an automated wizard for setting up the tolerances for gap allowance/disallowance and stitching of the surface to ensure a water tight solid.. I then ran a fillet on the edges we are discussing here.. It ran the same size as the MoI file I presented and also created the exact same twisted patch with the larger radius!!! (Than really fits with that geometry)





The red is the underside of the corner patch rolling over onto itself and folding upwards..

At this point, there will be some complicated "Help me fix this area" tools, asking questions that I need to know the answer to, to try and create something there..The help me tools may not exist for the issue and then I will need to know what to do. It may be a week long process to finish just this one component of the model. Then, the very next issue starts all over again... Maybe a months work or more???

Anyway.