Sparing Hu Songqi wasn't a spur-of-the-moment decision.
Some people have a character that's both timid and obstinate. They are destined never to settle down, always making trouble.
Hu Songqi is one of these people. Once she was sure that Link wasn't in the provincial city, she wouldn't be contented being so exploited, and would surely start to stir the pot in secret.
What Link wanted was for Hu Songqi to cause trouble, thereby giving him more clues about ancient witchcraft and evil witchcraft.
As for Hu Suqi, Link didn't fully believe what she said.
Even though he felt a bit guilty towards his eldest brother, Link still used some tactics to guide him to confess his love to Hu Suqi.
Whether Hu Suqi accepted it or not, it would serve as a type of restriction.
As for whether it would work or not, that could be seen later on; there was no need to rush to a conclusion.
The one who entered the Lotus Heart Boat was a clone.