Hi Quoc,
1/ Hi Clive,
You wrote:
The speaker thinks that more than one is correct. (5)
IMO, (5) is wrong. The correct sentence is:
The speaker thinks that more than one are correct. (6)
How about your opinion? Swan's Practical English Usage (Section 431) notes that 'more than one is followed by a singular . . . .verb'.
2/ Is this sentence correct?
What are correct? It sounds very odd to me. I'd say 'which are correct?'
Best wishes, Clive