I typically only hear of the term "flamethrower" in a weapons context so yes, I'd say that it has to be a weapon. Yes, you can have a noncombat device that projects a flame but those are typically called something else (like "burner" or "torch"). I'd expect most people to first think of a weapon when they hear "flamethrower".
And I would assume that your device's flame is still controlled and directed – it may have some spread but you still choose where to point the device even when it's active. You probably also have a means of turning the device off, offering further control. So your device fits the definition, even if it might be crude.
An incendiary grenade would be an example of a device that offers no control or direction. Once it goes off it releases all the fire everywhere within range. Another example would be a burning gas well – it might project its fire in a fairly predictable fashion and in a clear direction (up) but you can't easily turn it off or point it somewhere else.
Nope, they just become less predictable. Which is why in some parts of Germany you can't build as much as a garden shed without having EOD check the land first. In the more heavily-bombed areas it's not unusual to hear on the radio that you're to avoid downtown today between 10 and 12 because they're disarming a 500-pound bomb they found during roadwork.
And yes, the fact that an unstable bomb capable of trashing a city block is mundane nicely illustrates war's potential to fuck things up for generations.
Japan might want to get that land under and around the airport checked. There might be some other surprises hidden down there.