The flow preferences are used to select WAN1 and WAN2 for outbound traffic, They are not used to somehow select the IP range. That is purely the NAT config. But If you have a NAT config for WAN1, then you typically want to make sure that outbound traffic is also going to WAN1.
On the one for one NAT on interface 1. it is a completely different subnet than the Wan interface.
How is the one for one NAT 126.96.36.199 going to know what its default gateway is? Unless Comcast is programming a sub interface on the port form their device. you would still need to tell the 188.8.131.52/29 subnet what its default address is.