How to set up the sous vide bath for guineafowl breasts?
One of the benefits of sous vide cooking is that you cannot cook food over the temperature set, if you set the temp to 70 degrees it's physically impossible for it to go over that. This means that if you cook it longer than it needs to get up to the desired temperature you won't dry it out. I would think for guineafowl that 120 minutes would be enough. As for temperature shock you'd have to go much higher than 70C to run into that, so I'd start with the water at your desired temperature. 

As for sauces guineafowl is very rich and savory, so something with acidity would help to balance it. I like your sauce idea and think it would work fine. Riesling is a good choice if you get one with a more mineral character than a sweet bomb, although a squeeze of lemon can supplement the acidity if needed.