Let's say I have {memnarch} on the battlefield, and I make my opponents {grave titan} into an artifact. Might I cast {sculpting steel} to make a copy of it, at the end of the turn do I keep my copy? Does it matter that my opponents grave titan isn't an artifact anymore?
nope your good. {sculpting steel} isnt continually checking what it originally targeted. you will have an artifact copy of grave titan permanently.
Yes you can make the copy. Yes you get to keep it. The {Grave Titan} stays an artifact.