이건 거에요? is not complete sentence. It is 'is this?'. So awkward. And you know the rest of them well

이 책은 누구 거예요? 이거 마이클씨 거예요? 이거 제 거예요. Those are perfect sentences. You can use them in both spoken and written Korean. But 이건 거예요? is not a complete sentence. It is just a pattern that you can make sentences using with. So you need to add a word of owner like 내, 니, 엄마, 친구 to complete your sentence. I wouldn't use 이거 당신 거예요? to a stranger because it may sound rude because of the word 당신. I would say 이거 혹시 아저씨 거예요?, 이거 혹시 아가씨 거예요? and things like that. But be aware that you could call your spouse by 당신. In this case, you can say 이거 당신 거예요? without any rudeness.