here is a basic plan drawing of the bandstand idea.
the bents are indicated by solid lines and the purlins are indicated by dashed lines.
posts are indicated by brown circles and my main concern would be the red circle. Is it possible to build this frame with no post at the red circle?

The half octagon I'm thinking would be done with either a post in the middle(the musicians would not like this) or use a boss pin with principal rafters coming in at different elevations. I realize I should be talking to an engineer at this point but wanted to throw it out to the group and see if anyone might be familiar with a frame like this.