I've never been to Baden - I assume that it's the place half way between Auray and Vannes?
If it's the Baden that I've seen on the map, I think this will be a good location. You'd be away from the busy presqu'il de Rhuys, but near enough to nip across to Port Navalo and enjoy lunch in the quayside cafe without worrying about parking on a busy summer's day.
I wouldn't worry about finding about slipways in advance, as it's a bit like the Hamble - lots of slips like Swanwick, Hamble, Warsash, etc... And the French look upon it as a RIGHT to be able to launch a boat and go fishing or maintain their lobster pots. And in fact I've had a quick surf (google again) and found you a slipway at Port Blanc, less than 5km away: Port Blanc, Morbihan
- what a superb web site!!! - look at the link on the left ("La vie du port"), and then click on "Le port en image" when the sub-menu drops down. the first thumbnail shows you that the only problem you'll have is finding space amongst the dinghies.
Now, if you're considering taking a SIB instead of that lovely 5m RIB of yours, I reckon you shoud do a swop with me so I can take your RIB on holiday with me and you can take my SIB!
As far as journey times go, I'm not sure what it's like from St Malo, as we normally go on the Plymouth-Roscoff route. This is partly for personal reasons, as we meet friends & family in Plymouth, and partly to do the driving on "familiar" UK roads, than have a short journey in France. We've learnt to keep to the dual carriageways in Brittany for the arrival/departure journeys, as they are reliably quiet, fast and easy, and found that attempting to go cross-country can be slow and tiring (A-roads can be more like B-roads) . There are no toll roads in Brittany. We've also found that the Plymouth-Roscoff ferry times work well - outward journey at 3pm Saturday allows enough daytome travelling with a reasonably early start, and the 10pm arrival at Roscoff means that midnight to 1am arrival is sort of ok, but I guess that with children and going to a camp site, it could be unacceptable. An overnight en route could be better - we usually use Formule 1
or Premiere Classe
as they're really dead, dead cheap (~£20 a room per night) and they're all out of town (usually on an industrial estate) so no problem parking, leaving luggage in car, or even leaving a trailer hitched. (Alternatively Ibis Hotels can be useful. There's one at Landivisiau which is handy for Roscoff. Not useful for much else, as Landivisiau is the most boring place in France, IMHO.) Coming back, we usualy take the 11pm sailing (allows time for a leisurely journey back up country) and book cabins. Arrival back in Plymouth is about 6am. And last time we did this there was a cabaret on the boat with an excellent illusionist.
Of course, we will expect a FULL rib.net report on your expedition!