A couple of basic points whilst you wait for bevdeforges.

1) Is my wife eligible for social security and benefits as an EU citizen once resident in France for 3 months?

I know of no circumstances whereby any EU citizen is automatically accepted into the French health system by the simple expedient of being resident in France for 3 months. I don't know here you have got this information from. Are there any reciprocal health arrangements between Finland and France she could benefit from ?

2) If not, but I am as a non EU person with a carte de sejour, is it means tested? and if so, is the means testing determined by prior salary levels?

A Carte de Sejour is nothing to do with entry to the health system and AFAIK as a non EU citizen the only route for you into the French health system is via employment.