Thanks for all the input, folks! I really appreciate the thoughtful responses you've given me.
Here are my key thoughts after letting your comments percolate:
1. Priority number one is enjoyment. ColinsMum made a great point about risking frustration if DS suddenly finds himself with an hour where he isn't spoken to in English. A light touch is probably best at first. Maybe a better goal is to foster an appreciation and awareness of French than to achieve X fluency, which itself is an arbitrary goal.
2. I would like to phase in some French discussion with DS myself, but I do enjoy the richness of his English vocabulary. I think the opportunity cost of speaking English with me is too high at this point for me to begin French yet.
3. Native fluency was a bit of a specious goal with such low frequency of speaking French. I was able to pick up enough French in an hour each day at schooling kindergarten to become fluent, and our family has a history of multilingualism, so I have a feeling DS will respond like me (and mommin's DD with maybe 5-10 hours/week.)
4. How to develop a Canadian accent-- I have one, I'd choose a student teacher based on accent.
