More like 1.7B if you want to install a belt duration extender rig.
Yes.
7 days are about 3.3M m³, Income varies wildly depending on the ore of the moon. can be as low as 200M and as high as 1.1B.
No and yes. If your Athanor anchored successfully and you have online services, it takes at least 8 days to destroy the Athanor. However, when the Structure enters structure level reinforcement, the service turns off and the chunk pulling stops.
Depends entirely where you setup and if someone wants your moon or just wants to live in the system. https://zkillboard.com/ship/35835/losses/ doesn’t show that many high sec athanor deaths, considering how many there are.
The capital cost of the refinery is only the beginning - it will burn several hundred million ISK worth of fuel each month as well - exact amount will vary depending on the service modules you have installed.
Once the ore field is created, anyone can mine it.
Killing a high power structure in highsec is a lot of work for very little reward - people who attack you likely have some motive other than fun or profit. I expect that the high value moons worth fighting over are already taken so building a refinery on a medium value moon in some backwater system is probably fairly safe. That said, don’t fly what you can’t afford to lose.