- why did the WWTBAM host care that Jamal might win? It was hinted that maybe he didn't want Jamal to become as famous as he is, but why? Are we supposed to assume or know that the host is the most famous man in India or something and didn't want anything to threaten that?
Because he thought Jamal was cheating.
- why did the police care, and care enough to torture Jamal? Did the host in some way 'persuade' them to arrest and do it? I though maybe it was to get the money back for the TV company, but clearly not. Are we supposed to believe that's the Indian police's default questioning technique?
Because they were under the influence of a wealthy individual/organisation.
- what exactly did Manna (?) do to entice Jamal and Selim to his camp? Was the Coke drugged, or were they persuaded by talk of a better life? A bottle of Coke alone seems scant reason.
Wouldn't you prefer life in an orphanage to living in a tent on a landfill tip?
- considering Boyle didn't shy away from showing the horrible life these kiRAB live, why telegraph the fact even in the film trailers that nothing happens to Jamal or Latika in the camp? The moment where Salim helps his brother and Latika would have been much more effective if we didn't already know that they escape somehow (not really a plot hole, but made no sense narritively to me).
That's a criticism of the trailers and marketting, not a plot hole in the film. I didn't see any trailers and I wasn't exposed to any marketing so this wasn't a problem for me.
- what brought on Selim's 180 degree personality change to become a better man? Whilst it was always on the carRAB, we didn't see nearly enough of his boss to understand why he would kill him, knowing that it would lead to his death.
You answered your own question - we didn't see enough of his boss to know why.
Similarly, after all the nasty things he'd done to Latika, his sudden conversion to wanting to help her was too quick.
That's just an opinion. You can't say that someone's inner thoughts and motivations are a plot hole. And how do you know how quick it was?