One device really? Shall be allowed to use your ingress account for both the phone and tablet, not meant to do at same time but depends on where you play, if at home maybe prefer the tablet while phone at charge for example.

To the best of my knowledge, The one device is incorrect. You can have as many devices as you want. As long as there is the same account on all of them. Multiple phones have different carriers and different particularities. The benefit is on the carrier side (drift, signal) and not on the ingress side (inventory, extra hacking, etc)

Multiple devices are OK as long as you're the only one using all of them.

I think the "one device" is more referring to people who may try to play two devices at the same time (eg. Taking your mates account and phone to hack on).

I feel like this shouldn't be an unofficial rule, since it's already an official one.

I think some official rules will always get added to the unofficial ones because players feel they are very important. IMO, it does not matter if things as important as GPS spoofing and playing multiple accounts get added to the unofficial ones.

One device in use at any one time. Double wielding on the same account is not cool.

This should be a joined #1 with Spoofing.