These sorts of inverters are grid-tied so they turn themselves off when theres no grid to sync to (eg during an outage). My understanding is that's the main safety issue, and backfeeding while the grid is up is mostly a regulatory concern (as long as you have a modern meter that can tell the difference between electricity going in vs out)

Perfectly fine (at least here in the US) as long as your power meter is new enough to not double charge you for feeding back into the grid if your home draw drops low enough. Micro inverters are starting to really take off in modern solar installs to cut down on wiring distances since you can feed it into nearby AC circuits.

As long as the inverter feeds at most 800W. That's about 4A, on a circuit designed for 16A. You need a new meter, but the old analog meters can run backwards and you can continue using it until you power company replaces it. You do have to register the setup, but that seems to be a quick process. And if you lose power the inverter turns off