I think it's important to realize that MC doesn't want to do anything in Taekin's body without permission. His reasoning is solid... he would obviously try to help if Taekin was back in his own body and Heechan also keeping his body, but that's not what's happening. In fact, Heechan doesn't think he jumped in front of the car because of the bullying, that there must be something more to the story. If Heechan thought it was simple enough to take care of bullied to get his body back, he would've done it. But he also cares about Taekin and Taekin's mother, and I personally think that's really sweet of him. He also values Taekin's agency and doesn't want to permanently mess up his life. It makes sense, doesn't it?
I think it's important to realize that MC doesn't want to do anything in Taekin's body without permission. His reasoning is solid... he would obviously try to help if Taekin was back in his own body and Heechan also keeping his body, but that's not what's happening. In fact, Heechan doesn't think he jumped in front of the car because of the bullying, that there must be something more to the story. If Heechan thought it was simple enough to take care of bullied to get his body back, he would've done it. But he also cares about Taekin and Taekin's mother, and I personally think that's really sweet of him. He also values Taekin's agency and doesn't want to permanently mess up his life. It makes sense, doesn't it?