Android: route USA device traffic over France device mobile network
IP forwarding can be enabled with sysctl; iptables might have never been enabled / blocking forwarding or any traffics. (Speaking of that, the kernel you use needs to have the iptables components built.)
Another key is to either perform SNAT/MASQUERADE on the France device with the POSTROUTING chain of the nat table.
Also make sure you stop using AllowedIPs = 0.0.0.0/0 on the France device. Instead you list the subnet(s) the other devices are in.