Yes, it is. The Sabbath (7th day) would have begun at sundown Friday evening and lasted until sundown Saturday evening. At which time it becomes the first day. The evening comes first in Scripture, which is somewhat backwards to how our culture treats is today.
Why do you go to church on Sunday night? Why are church doors open Wednesday night? Did the pastor fail on Sunday morning?
Every day is a day of worship and learning... God set one day apart but that doesn't negate gathering at other times. It was tradition in that day to end the Sabbath with a meal and then some discussion often followed. In this case, Paul taught rather than discussed... that doesn't really have a bearing on the day.
No, and I don't share your view of the law. The mark of the new covenant is the law (same law) being written directly on the heart rather than one stone. To say "the law served it's purpose" would be the same as saying, "homosexuality was once a sin but now it isn't because the law served it's purpose." All the law is, is a collection of what God sees as sin and not sin. There are added judgements for when the Law is being used as civil law... as Israel used it until after Solomon.