Basically, MC is actually inhabiting his own body (proper reincarnation, woo) and a lot of things he heard about Jiang Wu (actually him lol) was nonsense. In the original timeline, he was framed by his mom (the first wife of the emperor of the enemy country) who lied to him saying he was actually the son of the enemy emperor. She also heavily ab*sed him since he was a kid to make him the perfect prince. He tried to seek help from the empress dowager and the empress at the time (who was his mom’s rival) but they didn't believe him and the ab*se got stronger so he became silent and lifeless. He ended up later saving the life of another prince who was drowning, and when they saved him (bc he let himself drown after), they saw the scars so they made the empress take care him. Still, his mom (Yao Ji, real name He Qi) continued to push him to be perfect by using needles and lying (stuff like his dad didn't love him) and later on she used a gu so everything she feels (like wounds) would also reflect on his body. That's why when the emperor died, he told everyone not to kill her even if she did something 'bad'.

Coming back to ML, he was actually MC’s childhood sweetheart. Other kids called him a savage when he arrived at the capital so MC taught him how to speak the proper language and stuff. But then he later had to leave to join his dad who was fighting on the border. After MC was framed (and there were rumors that he killed his brothers and he was a spy) and the country was in chaos, MC made ML promise to kill MC’s mom (so he wouldn't be associated with him and be branded a traitor) before eloping together. However, because of the mother gu in Yao Ji, he also died.

Later, ML became obsessed with taoism and buddhism because he wanted to find MC’s soul, but every true practician told him they couldn't find him because MC had no obsession. And it was true, he was just so tired he was actually happy to die after helping establish ML as a hero. But ML was veeeery stubborn and finally when he died (I think), he got offered to go to heaven but when he learned that MC wasn't there, he refused and insisted on finding him. In the end, the immortal-like guy told him to kneel at a mountain to have a chance of finding 'what he wanted'. But he also warned him that if he did so, he could never move until he found what he was looking for - and the chance of finding someone who didn't even want reincarnate (just to suffer again) was very slim.

So that's why MC got s**ked into his past body when he got close to a 'statue' (it was ML!). ML also went back to his own body, but since MC wouldn't remember him, he sealed all memories of MC so they would meet again 'fairly'.

He did say he was ready to do anything to get MC back.

It felt like the font end of the book was the author feeling their way towards a stronger outline, because there's a massive info dump in the back end. Taking any of these revealed points and seeding them earlier in the novel would have made this book a much better read.