You could, but why? You have an almost 25 year old truck. The 5k intervals have halped it last this long. How much can you really be saving by going 50% longer?

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.

There is fluid by which tire? All of them? Just one?

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.