Your experience may depend on which distro you use and how you install things. If you use a distro with a stable upgrade path such as Debian and stick to system packages there should be almost no issues with upgrades. If you use external installers or install from source you may experience issues depending on how the installer works.

For anything complex these days I’d recommend going with containers that way the application and the OS can be upgraded independently. It also makes producing a working copy of your production system for testing a trivial task.

I’n Windows it is not stored in a keyring but instead in the registry. This has basically the same security threat model as a local key file.

The ssh-agent on Linux will do what you want with effectively the same security. The biggest difference being that it doesn’t run as a system service but instead runs in userspace which can make it easier to dump memory. There are some other agent services out there with additional security options but they don’t change the threat model much.

My memory of the cp command is that attributes such as file times were transferred at the last step. I think this would make rsync safe in most situations where a system crash wasn’t involved.

Go with CAT6 and you should be fine if the total length of all 3 cables is less than 100m. CAT5 will work for the TV but CAT6 will work for more uses in the future and shouldn’t be too much more expensive unless your electrician has a bunch of CAT5 that they are trying to get rid of.

Edit: Also if you are thinking of wiring more locations the ideal solution is to bring all of the cables to a single location where you can connect to an Ethernet switch.