Okay I've tried a couple of things
1. I took off the extensions and rotated the filter wheel + camera. Note this isn't the same as rotating the camera only, but I will do that next.
2. Inside the extensions, I used black electrical tape to minimise any reflections. This isn't ideal because electrical tape still reflects so when I finish work tonight, I'll head to officeworks and get some black paper.
The result: Here's an SII sub.
The reflection has shifted. It also now has rotated. One thing I thought of is that the Flattener adapter has a male thread of 6mm in length. This screws into a female adapter 5mm thick.. so there's a 1mm overhang inside! I have no idea how to tape this up or black it out. Perhaps look for a different Flattener adapter with 4-5mm of thread? (Josh I might need your help again
)
The corner brightness issue is still there. So I know that's an issue somewhere inside the filter wheel or camera. I will check how it appears upon the next rotation.
I feel like progress is being made.. thank you all for your help thus far.