Let me start off by saying I’m probably not objective about the situation.
That is a fairly good assessment.
The concept of 6 feet of distance is an excellent place to start. You seem to be presuming that your MIL is not maintaining that at Mass. I would presume that hse is not only maintaining that, but likely 20 feet or more. If no one is showing signs of illness at Mass, the risk is somewhere between very low and negligible.
You do not say where you live. If you are in a larger city which has had a number of cases, some of the calculations of danger would be based on how long people have been self quarantining, the number of cases, and whether that has been flattening/going lower per day week over week.
If you live somewhere in an area of no known cases, you are over reacting. New York City? Ah, maybe not so much.
There has been one death in the smaller city I live in; I still go to the grocery store (3 in one day to find a small bottle of bleach!). I maintain distance, don’t go around touching everything, wash my hands…
While the virus is highly contagious, we are nowhere near the number of cases of flu this year (which I will grant is less contagious) and no one even made a peep about how to avoid the flu - except “get your flu shot”.
The likelihood of your changing your MIL’s mind sound a bit like between nil and none. A fairly simple statement of “Mom, I am concerned about your health, and about the health of everyone in this house. Please maintain the required distance with everyone at Mass; stay away from anyone coughing, and wash your hands thoroughly when you get home. And remember to pray for all of us when you are at Mass”.
There are 325,000,000 people in the US. across the entire US, there are about 123,000 cases (testing is now picking up on those who have contracted the disease but do not at this point need hospitalization - the majority).
Yes, there is a chance she would contract the virus; there is also a chance she could drop dead of a stroke or a heart attack. Again, if you are in a state or a city with a high level of infection, her risks are higher, but still low. If not, they are more likely minuscule.