I've never encountered "sinlessness" mentioned as a Divine attribute -- God is "All-Good," of course, which would necessarily include sinlessness, but would be so minor a part of it and so overshadowed by the Divine reality as not to be worth mentioning. Put another way, "All-Good" necessarily excludes sin (sort of tautological, or something, since sin is almost by definition that which is opposed to God!), but sinlessness doesn't come anywhere near implying "All-Good"!
No one that I've ever heard of has ever attributed omniscience to a creature -- to any creature. OTH, it's not at all hard to picture someone who knows far more than we do and who is nonetheless unimaginably far from omniscient.
“I’ve never encountered “sinlessness” mentioned as a Divine attribute “
I think it is a very major attribute. Were it not so, there would be no need that I can think of for heaven and hell. Heaven is a place of no sin; we can’t go there because we are sinners; Christ died to atone for our sins so that we could be declared righteous and so get into heaven. That’s a very central Christian doctrine.
Do you propose that God is a sinner? I can’t imagine how. We’d have no need of a Savior, then. Or God would need one.