I've not sat French as an external candidate, but I am for another subject which my school doesn't offer.
I enrolled through the NEC, and occasionally get email from other students taking French via distance learning. Apparently it is very tough, as you need to be able to expose yourself to spoken language to help with your listening as well as practice your speaking. I think that since your husband is French, you would have a huge advantage there - at school I only have contact with one native-French speaker as well. So I don't think you would be too disadvantaged in that respect.
Personally, I would go for it if you enjoy French and want to gain a qualification in it, and definitely if you can get onto the evening class - you will be able to get a lot of support off other people in a similar situation. Otherwise, it all depends on whether you feel you are up to self-studying - as it requires a lot, a lot of effort.
Sorry I can't be of more help in regards to the essay topics - I'm not really all too sure myself at the moment!