How serious is a mild facet arthropathy?

Not Serious. Meaning its not life threatening, however you will have pain. Recommend working on your core to improve pressure in your spine, however if this is not helping enough you may consider steroid injections into the facet joint or even radiofrequency ablation of the nerve to that joint for longer lasting relief. Some people have gotten good relief from PRP or stem cell therapy options as well.
It isn't. It is not serious and can be controlled with proper exercise, body mechanics, weight control as well as with smoking cessation. Sometimes, anti-inflammatory medication can be used.
It varies. In most cases it is no problem at all. Mild cases are often painless, but it can be difficult to determine if the arthropathy is causing a problem or not. Some physicians will inject the joint or the nerves to the joint to see if it relieves the symptoms (using low back pain).