Yes, I agree it is doable, but quite tight -and very stressful. And much more if you don´t know the system or don't speak French (and remember, there is a connection to Montparnasse).
If you choose train, don't board withouth a ticket. You can't purchase it on the train, and you risk a heavy fine.
There are clear signs at the airport with the direction to the station. The people at ticket office are helpful, speak English, and are used to tourists.
Air France Buses are more convenient if you are not familiar with the train and metro, but I agree, too, that traffic is quite unpredictable.
Sorry, there is not an obvious answer to your query. I' d say, try it, but with a "zen approach": if you make it, good, if not, take a later train, or enjoy a night in this wonderful city.
Buen camino!