Very interesting... normally the Walrider would be a surefire victory due to its ability to attack from the inside. Doing so would enable most contestants to be torn apart, buuutttt... the thing is Jack's mind and power aren't his own, they both are controlled by Eveline. In the event where Jack ends up possessed by the Walrider, then Eveline isn't going to have any of that and is going to defend Jack, since Eveline considers Jack to be part of her "family". Recall that once a person becomes infected by Evaline's fungus to a great enough degree then both Eveline's mind and the victim's become connected. So, theoretically, if Jack ends up possessed by the Walrider, then Eveline would likely be able to tell something was wrong and would amp up the control she has over the cells in Jack's body, akin to a puppetmaster giving the string's a harder tug when something grabs onto it.
I'm not saying that Jack wins for sure, but I'm pretty sure Eveline's possession would grant Jack some form of resistance to the Walrider.