誰は先週に映画を見た人か？
Ka here is playing the role of "da/desu" stem. Since the items into a sentence don't have to follow any specific order, it's safe to say that 先週に映画を見た人は誰？ is a correct sentence. It is implying the existence of ka or desu but not showing it explictly. 先週に映画を見た人は誰ですか？ has the same meaning. It's a completely valid sentence. Look at state of beeing lesson ^^
As for the guy who asked
As for the guy who asked this
先週に映画を見た人は誰？
The normal sentence would be
誰は先週に映画を見た人か？
Ka here is playing the role of "da/desu" stem. Since the items into a sentence don't have to follow any specific order, it's safe to say that 先週に映画を見た人は誰？ is a correct sentence. It is implying the existence of ka or desu but not showing it explictly. 先週に映画を見た人は誰ですか？ has the same meaning. It's a completely valid sentence. Look at state of beeing lesson ^^