As others have said, the view from Carnation Cafe's patio is not an ideal view. You'll be able to see the tops of the tall floats (e.g., you can see Mickey on the ball at the end), but you'll have trouble seeing the "on-the-ground" elements (like the dancers, Lightning McQueen, etc.) - basically anything that is below the height of a person will be hard to see. I think Carnation's patio is a relaxing alternative viewing location, but not one I would recommend for first time viewers. Along with the folks standing to watch the parade, there are also the people walking down Main Street that will obstruct your view. Within the patio area, there are space heaters and umbrellas are that might obstruct your view.
In terms of requesting a patio table, it's hit-or-miss whether they'll allow you to wait for a patio table (note, not all patio tables have an equal view) or not. I've been there when were told we had to take the next available table, when we were able to request a indoor or outdoor table, but told we would have to wait longer, and we've been there when the CM at check-in asked us which we would prefer. So there is really no way to know in advance which situation you'll get. My guess is that if your reservation is at 8:30 pm, the best patio tables will already be taken with guests that are also going to wait for Paint the Night, so any patio table that comes open will be towards the back or off to the side.