Not at the moment. I would have liked to pass the target's modified "Invisibility level" to the SDR script alongside the distance, hasLOS, and targetAttacked parameters. However, the OBSE mechanism for calling user scripts has some limitations which prevented this. The same is true for the detector "Blindness level", so you're not seeing the AVU effects on blindness either. For the time being, you'd need to reproduce the Diminished Returns forumla in your own scripts. It'd be a huge pain, but I can't offer anything else atm.
No worries. Other than the Moth Priests, blindness isn't used anywhere in the default game or DLCs, and as far as I know, no mods use it either.
As for creating my own curve, I'm not sure it's worth the time. There is already a system to tweak the effectiveness of Chameleon in three separate categories. The simplest solution would be for an end user to decide at what point the 100% effectiveness should kick in and reverse engineer it. For example, it is very hard but not impossible to get 120 av points of Chameleon. At 80% effectiveness, that would put the SDR visual at an effective 96 points for the visual.