For zoom, unless the flash can communicate with the camera, you control the zoom yourself. For my own experience, I tend to bounce a manual flash on my K-5 (and my K-x before that) and use the zoom to control the amount of area lit by the flash, rather than use the zoom function in correlation to my lens choice. As far as I know, only PTTL flashes such as Pentax's own flashes, Metz flashes and recent Sigma flashes will zoom for you.
As for what focal length to use, here's how I see it: when bouncing, go wide if you want to have a larger area lit and have things be generally lit; go tele if you want to highlight your subject from the background. If I want to just set-and-forget, assuming the roof doesn't change height, I'll just set the zoom to the widest setting, point the head up, find my exposure and keep it there. I'll then use my shutter speed to control ambient light and/or motion blur. Here's an example of what I got using that technique: