You need a different Vet -- one that specializes in leg problems. They will have to x-ray the splint area to rule out a fractured splint bone. That would come from a kick or a hit to the splint bone just below the old splint. You are just guessing without good quality x-rays.
Pain can radiate from some injuries and coincidences make things totally seem like something else. You have to rule out the splint bone FIRST. Then, the suspensory ligament must be thoroughly examined with whatever diagnostic tools a good lameness Vet believes should be used.
And yes -- Navicular syndrome can be diagnosed in one foot or can be worse in one foot than the other. Just remember that Navicular syndrome is a lot like colic. It only refers to pain in a certain area. The cause of that pain can be one or more of many things. The cause of the foot pain must be determined, again, by a good lameness specialist.
By the time you mess around with call after call of a regular Vet, you will have spent as much money as if you went to a specialist right away and the chances are very good that you still will not know what you have. Meanwhile, your horse can have complications that are the result of having it go as long as it already has without letting it go any longer.