Nah I think the author is just taking the long way to justify why and how Soleile managed to cope waiting for protag reincarnation and how he became so devoted to him and why he's brother is so set in tearing them apart.
I think Ian will just assimilate the memory of his past life if he remembers. If he decide to see those memories as just memories of someone else, he will remain as his self. Otherwise and I think most probable is that he'll try reconcile his two selfs where Ian retains himself but also see his past life as something a kin to seeing it as his younger self very different to who he is as an adult. Ian and Brian have almost opposite personality. Ian's personality and character is quite strong too so the likeliness of him being all grim, suspecting and torn between his loyalty to blonde cardinal etc to become Brian is hard to imagine.
Im starting to think that the author never really saw ian as a separate character, hes more so a vessel for brians spirit ☹ ive been waiting for the flashbacks to finish but i think once he gets his memories back ian will cease to exist