1. The outer limit for the ride to the airport has been an hour. As far as i can remember it always turns out to be less than that. Yes, your bound to have slightly less traffic on a Sunday evening.

2. Yeah, as mentioned earlier with Mt. Lavinia it's pretty much the hotel and not much to do in terms of strolling around in the nearby areas. Whereas, with the Galle Face hotel, since it's in the heart of the city and is very near the beach promenade, you could spend some time strolling around there. Your recollections are correct. Would be best i think to engage a taxi for the entire duration of this run into town. Should be able to strike a deal for the same on arrival at CMB.

Hope this helps..have a nice stopover in CMB..