-- Last Updated: Nov-17-12 7:41 PM EST --
You wanted to know what the experts think, but in the meantime, I'll tell you what I think.
Though most people do without them, foot braces are really nice if you paddle seated. In fact, I found out by accident that a seat that's slanted for kneeling makes a really good seat when using a foot brace, so you could set up the boat to do both sitting and kneeling by mounting the seats on a slope AND adding footbraces.
Regarding kneeling, chances are good that the seats (yours anyway) need to be a little higher for kneeling, and of course it should slope forward. You'll probably find that raising the seat an inch or so makes your feet fit underneath a whole lot easier. Choosing the right footwear helps too. Choose foot wear that allows your feet to extend so that the tops of the feet can easily face the floor of the boat without all the contact being the just at the tip of the shoe's toe area. Normal boots made for walking usually don't work so well for that, but I find that paddling boots do. Kneeling with just a simple kneeling pad pretty-much eliminates any tendency for you to slip around in reaction to paddle strokes for normal paddling. Start horsing the boat around a little harder and glued-in kneeling pads are nice, but what I've done is to sew sections of pool noodle into a sleeve at the front edge of the kneeling pad, and that totally prevents me from sliding forward without changing anything about the boat. I also install a few "shower strips" on the floor to keep the kneeling pad from slipping.
As far as "how much moving around" is acceptable, you'll find you can move around and tip the boat quite a lot. The boat won't tip over when leaned as long as you keep your body vertical. Leaning the boat by anchoring your knees and leaning your whole body is another story - so don't do that. Basically, remember to keep your head over your center of support. You'll have a lot more margin for error if that center of support is the boat's floor (kneeling), but if it's the seat, sharp leans have more of a vague feeling for both control and the limit for leaning. Once you get the feel for it, you WILL be able to lean your upper body well outside the canoe when doing eddy turns or other sharp maneuvers, letting the dynamics of the paddle-water connection hold you up, but there's no need to rush into that.