I agree with you that they may have had preconceived notions about how Tyler and Hamza would do, and might have expected Helen to win going in, but I'm not sure I see it with Bill. Just my personal opinion, but I always find it confusing when people say the producers didn't foresee Bill winning. Idk, but my first thought when I saw he'd been announced (after 'holy tit, I can't believe Bill Bailey's doing Strictly!) was 'lol, well that's a damn shame for everyone else, he's already won.'
Maybe in a normal length series he wouldn't have won, as perhaps the audience would have gotten bored after he plateaued and he'd have gone out somewhere around the week 9-11 mark, but with the shorter length, it's hard to see how the producers could have been stupid enough to think a celebrity as big as him wasn't winning, unless he was Anne Widdecombe level bad, and surely they knew he wasn't from the audition. Also hard to see how they thought Janette would win with a ringer. Even if male ringers aren't generally as unpopular as female, they still rarely win. That said, I do wonder why Oti didn't end up partnering Jamie like she was originally supposed to? That's the only thing that makes me question it a bit.