Hi Peter,
So while a few more measurements will really confirm this, it looks like it is a combination of 4) + a slightly different 1) and 3).
The camera has to much QE for the exotic refractor hmmm.... I guess I'll ditch the camera and find a worse one...
But seriously have you considered a catadioptric? Although if a simple bandpass filter fixes the problem, well that is the best outcome for your wallet.
Best,
EB