As I continue digging into the Coningsby quagmire, it may be that a fairly simple answer exists. John's very existence has been brought into question by a couple of websites. Fabpedigrees has the Coningsby family pedigree and suggests that the existence of John, who died in 1216 [or 1266] and his father Roger may be "fictional." I believe that this may be based on a Google Groups post that suggests Coningsby may be tied to the Marmion family which held Coningsby and Horncastle. It is further suggested that John de Coningsby's father may have been a Roger de Marmion.
I have no idea if the suppositions are close to being fact. As I suggested in an earlier post, sometimes you can trace a family line back too far! 😝