The first attack that came from Ephthal where he casted level 6 chantless magic was very questionable on his part although he wanted to not be a high-profile figure. I think what happened here was his desire to see his disciple's progress outweigh his desire to remain low-key. Regardless if he realized what was occurring in the middle of the battle, him firing off high level magic chantless would give him a lot of popularity, and even if he wanted to stop the battle, it would be hard to stop because of how heated it is already.
The only disappointment here was Merlin not realizing that you can't just battle at high levels without any experience regardless of whether or not the memories remained. But my expectations of manga/manhwa/manhua are as low as the quality of them that I read anyway...
“he's like the splitting image of him” oh jeez I wonder what made you think that. Could it be that they look identical, have the exact same personality, knowledge, and skill? Nah, I'm sure it's just a coincidence.