Cursed energy isn't concepts. You are conflating the cause with the effect. Cursed energy might causes concept manipulation, but it isn't concepts in itself. Fran can negate it perfectly fine before the stage where it causes those things.
I'm not. Cursed energy is a fundemantel essence in the JJK curse that is not only responsible for the esoteric effects in the world, but the actual essence of ontology including things such as humans, curses, and animals.
If you're positing that any old powernull that works on "curses" that aren't synonymous with curses in JJK, then that's just plainly an quivocaiton fallacy/NLF. The potency needs to be there as even the most basic simple domain in JJK can powernull the most broken techniques, that's a testament to barrier powernull, not a weakness of limitless.
It's automatic, not passive. If it were passive, Gojo would at all times have infinite distance to all objects and could never touch anything.
No, the selectivity of limitless is completely separate from its passiveness. Gojo can already manually keep it on for dozens of hours straight, his automatic state keeps it passive for him.
It works like Eldemade's scans show, which is it automatically identifies a threat and then automatically activates the ability. Neither is there reason to assume it does so instantaneously, nor can Gojo even pick up on the threat in this case.
You are looking at scans from when gojo first awakened that application of the technique, a decade into the past. Once again, even when Gojo was doing it manually, he was able to keep it successively for dozens of hours straight, hence why Toji had to literally follow them and wait until the end of their mission when Gojo finally let down his infinity. The new infinity is constantly upkept by him and automatically detects threats outside of his purview, this goes hand in hand with his RCT which works 24/7 without his will to upkeep the strain on his brain for constantly using his technique.
Once again, you are using feats from an 18 year old Gojo. Gojo's limitless can stop the radiation of heat, which is propagated through EM waves. He has also stopped intangible spirits from affecting him in a timeless realm, so his scope of things he himself can interact is also vast so I'm not really sure what you means by threats that limitless can't pick up on.