Hi Adam,
all the larger rapids (Portefeuille, Slalom, Portes de l'Enfer) can be scouted/walked easily. I am not sure about the other ones, but at medium-low water level they are class 2 - 3(-). Cascades would be a long walk... A good option to avoid the first portage would be to put-in at Aqua-aventure (first R2-3 after the slalom). there might be a small parking fee.
I'll leave the "how good does your roll need to be" to Bill and whoever will do the rescue...
it can be paddled in a playboat at any levels; the lines are relatively easy. however, there are no good playspots (except at the top of Cascades rapids at high level, a one-shot deal) and many, many manoeuvers to do if you want to add some challenges. and there are a few green waves (e.g. bottom of the slalom) that are much easier to surf with a longer boat. a river runner is more adapted to the river, but you can still go down in a playboat...
I would not practice rolls at the take-out. there is no big eddy there and the current is relatively strong with waterfalls a few hundred meters downstream... not usually a place to be concerned, but practicing rolls there is not the best idea. and the water is usually quite discusting at the take-out because the last flat is surrounded by farms... unless your into mud-mask therapy? Aqua-Aventure would be a much better place to practice. and there are two nice little rapids with a pool a the bottom too.
