A: Absolutely. The entire game becomes bright, though the Nether’s ambient red fog remains.
A: Possibly. While it’s not X-ray, automated anti-cheat systems sometimes detect abnormal lightmap data. Use only on servers that explicitly allow visual modifications. minecraft 1.12.2 fullbright texture pack
Just remember: It flattens the visual experience and won’t save you from a zombie sneaking up on you in the dark. Use it wisely, and respect server rules. A: Absolutely
A: Yes, but disable OptiFine’s "Custom Colors" and "Internal Shaders" to prevent conflicts. and respect server rules. A: Yes