I'd suggest, as others have, getting some professional help.
Kids manipulate because they've found its what works for them. I grew up as a manipulative kid and didn't realize it till I was an adult and still manipulating. I was the youngest and quietest of 4. My siblings were all older, louder and more willing to speak up. I found I had to use other means to get things, hence the manipulation.
Also, when I was maybe 5-ish I'm told I suddenly stopped eating. I'd eat half a bologna sandwich a day and nothing else. I was in and out of hospitals getting all kinds of testing. After a year I snapped out of it. No cause was ever found.
The doctors told my parents to give me anything I asked for any time I wanted to eat, since I wasn't eating enough.
I was a picky eater, still am at 46. My parents tried the "no leaving the table till you finish everything" tactic all the time. Issue was, I just actually didn't like those things, and still don't.
As a parent you, no doubt, want to be able to solve all this on your own, but that's not what's always best. Sometimes it's better to go to outside sources for help and ideas.
So find a counselor or psychologist. Get a doctor involved. Maybe a nutritionist too. Find out what you're dealing with and the best plan on how to go about it.