Most splitters aren’t even that. They connect both devices over the same 4-pairs. So it only works if one of the devices is powered off.

If you read the manual, you’d see you can use the 2.5Gbps port for either LAN or WAN.

Fiber can be oversold just like any other internet connection type.

Comcast might work. Most of their infrastructure has small battery backups, but will eventually go down after a few hours. For long term outages they will send out techs to hook up generators to bring the infrastructure back online. But for outages that last for less than a day, the internet will stop working after a few hours.