In regards to defaulting to the gateway page, I'm pretty sure the only way to do it is by changing the default modem IP address prior to making the changes which doesn't quite achieve what you were hoping to do there. Unfortunately though, there's no way that I know of personally on how to prioritise if theres a conflict in IP addresses. This is less to do with the modem itself and more to do with knowledge on networks.
Hopefully that helps narrow things down a little though!
Yep, should be under "Home Network">"LAN Interface">"LAN Interface Settings">"IP Address">Set new IP address then click "Save". I can't remember if the modem will reboot though and if it doesn't, maybe do that via power button, 2 mins etc etc