Do you think the Lord of Light would be the most omniscient being? On the same note, what powers would a Muse of Light have?

They could be Omniscient but maybe not even necessarily. If you mean not having dark spots like Scratch, maybe? As for the powers of the Muse of Light, though, they’d be able to Command Light- Information and Knowledge are a very powerful tool, so being able to have absolute control over these would make the Master Classes quite potent with this. For the Muse, they may subtly edit well known information or facts to leave messages or bring issues to light that someone or something is trying to keep hidden and secret. Commanding Relevance means whatever they’re concerned with would be, objectively, the most important thing in Paradox Space, and they may also be a big source of relevant events themselves. On top of that, Commanding Light itself would give them control over visual hallucinations, cloaking stuff and visual illusions in general. They’d also, 100% sure, no doubt, have Abraxas as their Denizen.