So I'm just using the comic book parameters to "fix" these comic book-based movies LMAO. And it works, it's proper and legit. Within the rules established by the (DOFP) movie ON-SCREEN, if you're willing to "buy" the concept of a serum that makes Xavier walk again but inhibits his powers, it's not a stretch to think to an IMPROVED, upgraded version that makes him walk AND use his powers at the same time.