They couldn't use Hypnotize to mold Silver Mirror into their perfect tool, but they didn't need to do anything to shape him. He understood his situation perfectly.
Silver Mirror doesn't want to die. As a coward who left his sister behind and escaped for his life, the one thing he cares about the most is his life. So he doesn't need their encouragement to do what is necessary to preserve his life.
Besides, they don't need Silver Mirror to become a perfect tool for them. They only need him to help them reach the city and give them some information to settle down.
Now that he has been subdued, they can rest assured that he won't send them to hell or somewhere dangerous when they use his gateway.
This assurance made them give him spiritual energy through the pathway of the slave so that he could recover quickly.
The physique of epic beings is very special. If they are not killed, as long as they still have spiritual energy, they will recover no matter how injured they are.
