You will need internet to download games, but it can operate in offline mode as well if the game supports it.
You do need an account, but it can be a Meta account not connected to your Facebook if your choose.
They are not hackable in any way and there are no custom firmwares. In fact, the software side of these devices is incredibly complex (tracking, lens correction, very low latency rendering and control pipelines) and replicating the work that FB has done would be an effort for a large team of VR engineers. If you dig into some of the details of how they work you will realize they are a technological marvel to rival the iPhone.
There is a homebrew scene however, on sidequest.
Maybe, but Halflife Alyx is almost worth the cost of entry on its own
Don't know about Quest 3, but probably the same.
There are some additional standalone body trackers, like SlimeVR, Sony mocopi, or HaritoraX, where you strap the hardware widgets on and they communicate inertial measurements with each other and your device without needing any base stations. But, since there's no OS-level support for them on Quest, support is app-by-app and usually requires a second device as a go-between (like a phone running the mocopi app).
PCVR makes it easy to use a ton of trackers and they're much more precise, but they require base stations (IR cameras on mounts you set up around your play space) since each one is basically a little IR transmitter with its position computed by the PC. If you see any "full body trolling" videos on Youtube it's generally people using these, since you can do strange things with them (handstands, pogo stick bouncing, multiple people sharing the same physical space, other Youtube creator nonsense) without the tracking getting confused or desynced.