Once again, thank you for your previous ratings. To answer your questions:
1. Your citizenship makes no difference. Your residence does, in the sense that in the case of a trial, required to impose the obligation of support on you, she would have to summon you internationally, which is rather harder than nationally. However, the law itself applies regardless of citizenship. The same answer applies to anyone else: this part of the French Civil Code applies internationally as well, because the "protected person" is French living in France;
2. Your son from your first marriage is not liable under any circumstances. This being a personal obligation, even in an inheritance situation, he would not inherit your obligation to provide support. On a side note, he may be a good recipient of your assets since, if transferred with legal title to him, they would then not be touchable in his hands;
3. Your primary residence's value is not taken into account for the purpose of computing what you would have to pay. Only your other assets and income are. So in this sense it is not directly liable. However, if you are obliged by the court to pay something, and you do not pay, there is a right of the creditor (i.e. the mother-in-law) to pursue any assets you have for the payment of arrears. So in this sense it could be in danger, but only if you do not pay out of your income. Furthermore, before pursuing the residence, she must pursue any other assets, and you will have plenty of opportunity during such a process to provide alternative payment. In other words, the possibility of reaching this point is very remote.
I hope these answers will also be useful and look forward to your rating.
With my best regards,
Dr I L Vlad